drm/cirrus: allow 32bpp framebuffers for cirrus drm

This patch allows framebuffers for cirrus to be created with
32bpp pixel formats provided that they do not violate certain
restrictions of the cirrus hardware.

v2: Use pci resource length for vram size.

Signed-off-by: Zach Reizner <zachr@google.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>