drm/vkms: Remove set but not used variable 'vkms_obj'

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

drivers/gpu/drm/vkms/vkms_plane.c: In function 'vkms_prepare_fb':
drivers/gpu/drm/vkms/vkms_plane.c:144:26: warning:
 variable 'vkms_obj' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit
8ce1bb0b ("drm/vkms: map/unmap buffers in [prepare/cleanup]_fb hooks")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1543634444-186448-1-git-send-email-yuehaibing@huawei.com