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 <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -339,7 +339,7 @@ static bool start_sync_sw(
 	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)
 {
@@ -353,7 +353,7 @@ void dce_i2c_sw_engine_set_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)
 {
@@ -397,7 +397,7 @@ bool dce_i2c_engine_acquire_sw(
 
 
 
-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)
 {
@@ -440,7 +440,8 @@ void dce_i2c_sw_engine_submit_channel_request(
 		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)