modesetting: Fix 16 bit depth/bpp mode

When setting DefaultDepth to 16 in the Screen section, the current
code requests a 32 bpp framebuffer, however the X-Server seems to
assumes 16 bpp.

Fixes commit 21217d02 ("modesetting: Implement 32->24 bpp
conversion in shadow update")

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>