drm/amd/display: remove unused variable 'dc'

GCC reports the following warning with W=1:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_psr.c:70:13:
warning:
 variable ‘dc’ set but not used [-Wunused-but-set-variable]
    70 |  struct dc *dc = NULL;
       |             ^~

This variable is not used in function, this commit remove it to
fix the warning.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>