Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/vmwgfx/vmwgfx_fb.c: In function 'vmw_fb_init':
drivers/gpu/drm/vmwgfx/vmwgfx_fb.c:645:29: warning:
 variable 'fb_offset' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/vmwgfx/vmwgfx_fb.c:645:19: warning:
 variable 'fb_depth' set but not used [-Wunused-but-set-variable]

They're not used any more, so can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Deepak Rawat <drawat@vmware.com>