drm/amd/display: Remove set but used 'temp'

Addresses the following gcc warning with "make W=1":

In file included from drivers/gpu/drm/amd/amdgpu/../display/dmub/src/../dmub_srv.h:67:0,
	from drivers/gpu/drm/amd/amdgpu/../display/dmub/src/dmub_dcn21.c:26:
drivers/gpu/drm/amd/amdgpu/../display/dmub/src/../inc/dmub_cmd.h: In function ‘dmub_rb_flush_pending’:
drivers/gpu/drm/amd/amdgpu/../display/dmub/src/../inc/dmub_cmd.h:795:12: warning: variable ‘temp’ set but not used
 [-Wunused-but-set-variable]
    uint64_t temp;
                ^
In file included from drivers/gpu/drm/amd/amdgpu/../display/dmub/src/../dmub_srv.h:67:0,
                 from drivers/gpu/drm/amd/amdgpu/../display/dmub/src/dmub_dcn30.c:26:
drivers/gpu/drm/amd/amdgpu/../display/dmub/src/../inc/dmub_cmd.h: In function ‘dmub_rb_flush_pending’:
drivers/gpu/drm/amd/amdgpu/../display/dmub/src/../inc/dmub_cmd.h:795:12: warning: variable ‘temp’ set but not used
[-Wunused-but-set-variable]
   uint64_t temp;

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>