drm/amd/display/dc/dce/dce_i2c_sw: Make a bunch of local functions static

Fixes the following W=1 kernel build warning(s):

drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_i2c_sw.c:342:6: warning: no previous prototype for ‘dce_i2c_sw_engine_set_speed’ [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_i2c_sw.c:356:6: warning: no previous prototype for ‘dce_i2c_sw_engine_acquire_engine’ [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_i2c_sw.c:400:6: warning: no previous prototype for ‘dce_i2c_sw_engine_submit_channel_request’ [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_i2c_sw.c:443:6: warning: no previous prototype for ‘dce_i2c_sw_engine_submit_payload’ [-Wmissing-prototypes]

Cc: Harry Wentland <[e-mail]>
Cc: Leo Li <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Cc: "Christian König" <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Lee Jones <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/dce/dce_i2c_sw.c

···
 	return false;
 }
 
-void dce_i2c_sw_engine_set_speed(
+static void dce_i2c_sw_engine_set_speed(
 	struct dce_i2c_sw *engine,
 	uint32_t speed)
 {
···
 		engine->clock_delay = 12;
 }
 
-bool dce_i2c_sw_engine_acquire_engine(
+static bool dce_i2c_sw_engine_acquire_engine(
 	struct dce_i2c_sw *engine,
 	struct ddc *ddc)
 {
···
 
 
 
-void dce_i2c_sw_engine_submit_channel_request(
+static void dce_i2c_sw_engine_submit_channel_request(
 	struct dce_i2c_sw *engine,
 	struct i2c_request_transaction_data *req)
 {
···
 		I2C_CHANNEL_OPERATION_SUCCEEDED :
 		I2C_CHANNEL_OPERATION_FAILED;
 }
-bool dce_i2c_sw_engine_submit_payload(
+
+static bool dce_i2c_sw_engine_submit_payload(
 	struct dce_i2c_sw *engine,
 	struct i2c_payload *payload,
 	bool middle_of_transaction)