drm/amd/display: Increase precision for backlight curve

[Why]
We are currently losing precision when we convert from
16 bit --> 8 bit --> 16 bit.

[How]
We shouldn't down convert unnecessarily and lose precision.
Keep values at 16 bit and use directly.

Signed-off-by: Anthony Koo <[e-mail]>
Reviewed-by: Aric Cyr <[e-mail]>
Acked-by: Leo Li <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/modules/power/power_helpers.c

···
 };
 #pragma pack(pop)
 
-static uint16_t backlight_8_to_16(unsigned int backlight_8bit)
-{
-	return (uint16_t)(backlight_8bit * 0x101);
-}
-
 static void fill_backlight_transform_table(struct dmcu_iram_parameters params,
 		struct iram_table_v_2 *table)
 {
 	unsigned int i;
 	unsigned int num_entries = NUM_BL_CURVE_SEGS;
-	unsigned int query_input_8bit;
-	unsigned int query_output_8bit;
 	unsigned int lut_index;
 
 	table->backlight_thresholds[0] = 0;
···
 	 * format U4.10.
 	 */
 	for (i = 1; i+1 < num_entries; i++) {
-		query_input_8bit = DIV_ROUNDUP((i * 256), num_entries);
-
 		lut_index = (params.backlight_lut_array_size - 1) * i / (num_entries - 1);
 		ASSERT(lut_index < params.backlight_lut_array_size);
-		query_output_8bit = params.backlight_lut_array[lut_index] >> 8;
 
 		table->backlight_thresholds[i] =
-				backlight_8_to_16(query_input_8bit);
+			cpu_to_be16(DIV_ROUNDUP((i * 65536), num_entries));
 		table->backlight_offsets[i] =
-				backlight_8_to_16(query_output_8bit);
+			cpu_to_be16(params.backlight_lut_array[lut_index]);
 	}
 }
 
···
 {
 	unsigned int i;
 	unsigned int num_entries = NUM_BL_CURVE_SEGS;
-	unsigned int query_input_8bit;
-	unsigned int query_output_8bit;
 	unsigned int lut_index;
 
 	table->backlight_thresholds[0] = 0;
···
 	 * format U4.10.
 	 */
 	for (i = 1; i+1 < num_entries; i++) {
-		query_input_8bit = DIV_ROUNDUP((i * 256), num_entries);
-
 		lut_index = (params.backlight_lut_array_size - 1) * i / (num_entries - 1);
 		ASSERT(lut_index < params.backlight_lut_array_size);
-		query_output_8bit = params.backlight_lut_array[lut_index] >> 8;
 
 		table->backlight_thresholds[i] =
-				backlight_8_to_16(query_input_8bit);
+			cpu_to_be16(DIV_ROUNDUP((i * 65536), num_entries));
 		table->backlight_offsets[i] =
-				backlight_8_to_16(query_output_8bit);
+			cpu_to_be16(params.backlight_lut_array[lut_index]);
 	}
 }