drm/vmwgfx: Add an option to change assumed FB bpp

drm/vmwgfx: Add an option to change assumed FB bpp

commit 04319d89fbec72dfd60738003c3813b97c1d5f5a upstream.

Offer an option for advanced users who want larger modes at 16bpp.

This becomes necessary after the fix: "Work around mode set
failure in 2D VMs."  Without this patch, there would be no way
for existing advanced users to get to a high res mode, and the
regression is they will likely get a black screen after a software
update on their current VM.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>