drm/amd/display/dc/core/dc_link_dp: Mark 'result_write_min_hblank' as __maybe_unused

It looks like it could be used inside the DC_TRACE_LEVEL_MESSAGE() macro.

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

 drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c: In function ‘dpcd_set_source_specific_data’:
 drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:4403:18: warning: variable ‘result_write_min_hblank’ set but not used [-Wunused-but-set-variable]

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/core/dc_link_dp.c

@@ -4406,7 +4406,7 @@ void dp_set_fec_enable(struct dc_link *link, bool enable)
 void dpcd_set_source_specific_data(struct dc_link *link)
 {
 	if (!link->dc->vendor_signature.is_valid) {
-		enum dc_status result_write_min_hblank = DC_NOT_SUPPORTED;
+		enum dc_status __maybe_unused result_write_min_hblank = DC_NOT_SUPPORTED;
 		struct dpcd_amd_signature amd_signature = {0};
 		struct dpcd_amd_device_id amd_device_id = {0};