drm/amd/display: Fix variable dereferenced before check

Fixes the following smatch warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:311 dc_dmub_srv_p_state_delegate()
warn: variable dereferenced before check 'dc' (see line 309)

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>