gma500: support 1080p

The problem in console mode is lack of linear memory. We can solve that by
dropping to 16bpp. The mode setting X server will allocate its own GEM
framebuffer in 32bpp and all will be well.

We could just do 16bpp anyway but that would be a regression on the lower
modes as many distributions don't yet ship the generic mode setting KMS
drivers.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>