drm/amd/display/dc/dce/dce_i2c_hw: Make functions called by reference static

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_i2c_hw.c:438:35: warning: no previous prototype for ‘dce_i2c_hw_engine_wait_on_operation_result’ [-Wmissing-prototypes]
 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_i2c_hw.c:505:6: warning: no previous prototype for ‘dce_i2c_hw_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: Lewis Huang <Lewis.Huang@amd.com>
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_hw.c

@@ -435,7 +435,7 @@ struct dce_i2c_hw *acquire_i2c_hw_engine(
 	return dce_i2c_hw;
 }
 
-enum i2c_channel_operation_result dce_i2c_hw_engine_wait_on_operation_result(
+static enum i2c_channel_operation_result dce_i2c_hw_engine_wait_on_operation_result(
 	struct dce_i2c_hw *dce_i2c_hw,
 	uint32_t timeout,
 	enum i2c_channel_operation_result expected_result)
@@ -502,7 +502,7 @@ static uint32_t get_transaction_timeout_hw(
 	return period_timeout * num_of_clock_stretches;
 }
 
-bool dce_i2c_hw_engine_submit_payload(
+static bool dce_i2c_hw_engine_submit_payload(
 	struct dce_i2c_hw *dce_i2c_hw,
 	struct i2c_payload *payload,
 	bool middle_of_transaction,