drm/vmwgfx: select CONFIG_FB

vmwgfx doesn't actually build without that.

It would be great if vmwgfx could switch over to the fbdev emulation
helpers, since those will take care of all this already. I guess one
reason to not do that was lack of defio support in the helpers, but
that's fixed now. If there's any other hold-up, we should figure out
what it is and whether it makes sense to address it.

Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470847958-28465-4-git-send-email-daniel.vetter@ffwll.ch