drm/i915: Allow frame buffers up to 4096x4096 on 915/945 class hardware

The 915 and 945 scanout engines can handle frame buffers up to 4096 pixels
wide. Pre-9xx hardware has an 8192 byte stride limit, and so we leave the
existing 2048 max in place.

I'm not sure why we limit the height to the same value; there's no intrinsic
hardware limit in the scanout engine.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>