drm/vmwgfx: depends on FB

vmwfgx uses framebuffer interfaces, so it should depend on FB.
Otherwise it has these build errors (e.g., when CONFIG_FB=m):

drivers/built-in.o: In function `vmw_fb_close':
(.text+0x97713): undefined reference to `unregister_framebuffer'
drivers/built-in.o: In function `vmw_fb_close':
(.text+0x97754): undefined reference to `framebuffer_release'
drivers/built-in.o: In function `vmw_fb_init':
(.text+0x97e1c): undefined reference to `framebuffer_alloc'
drivers/built-in.o: In function `vmw_fb_init':
(.text+0x9838d): undefined reference to `register_framebuffer'
drivers/built-in.o: In function `vmw_fb_init':
(.text+0x9842a): undefined reference to `framebuffer_release'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>