drm/vmwgfx: Do not allow invalid bpp's for dumb buffers

Dumb buffers allow a very limited set of formats. Basically everything
apart from 1, 2 and 4 is expected to return an error. Make vmwgfx
follow those guidelines.

This fixes igt's dumb_buffer invalid_bpp test on vmwgfx.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221022040236.616490-13-zack@kde.org