drm/mgag200: Reject modes that are too big for VRAM

A monitor or a user could request a resolution greater than the
available VRAM for the backing framebuffer. This change checks the
required framebuffer size against the max VRAM size and rejects modes
if they are too big. This change can also remove a mode request passed
in via the video= parameter.

Signed-off-by: Christopher Harvey <charvey@matrox.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>