drm: Fix DSC BPP increment decoding

Starting with DPCD version 2.0 bits 6:3 of the DP_DSC_BITS_PER_PIXEL_INC
DPCD register contains the NativeYCbCr422_MAX_bpp_DELTA field, which can
be non-zero as opposed to earlier DPCD versions, hence decoding the
bit_per_pixel increment value at bits 2:0 in the same register requires
applying a mask, do so.

Cc: Ankit Nautiyal <[e-mail]>
Fixes: 0c2287c96521 ("drm/display/dp: Add helper function to get DSC bpp precision")
Reviewed-by: Jani Nikula <[e-mail]>
Signed-off-by: Imre Deak <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/display/drm_dp_helper.c

···
 {
 	u8 bpp_increment_dpcd = dsc_dpcd[DP_DSC_BITS_PER_PIXEL_INC - DP_DSC_SUPPORT];
 
-	switch (bpp_increment_dpcd) {
+	switch (bpp_increment_dpcd & DP_DSC_BITS_PER_PIXEL_MASK) {
 	case DP_DSC_BITS_PER_PIXEL_1_16:
 		return 16;
 	case DP_DSC_BITS_PER_PIXEL_1_8:

include/drm/display/drm_dp.h

···
 # define DP_DSC_BITS_PER_PIXEL_1_4          0x2
 # define DP_DSC_BITS_PER_PIXEL_1_2          0x3
 # define DP_DSC_BITS_PER_PIXEL_1_1          0x4
+# define DP_DSC_BITS_PER_PIXEL_MASK         0x7
 
 #define DP_PSR_SUPPORT                      0x070   /* XXX 1.2? */
 # define DP_PSR_IS_SUPPORTED                1