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>