drm/vmwgfx: remove set but not used variable 'srf'

drivers/gpu/drm/vmwgfx/vmwgfx_surface.c:339:22:
 warning: variable srf set but not used [-Wunused-but-set-variable]

'srf' is never used, so can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>