drm: kill DRIVER_REQUIRE_AGP

Only the two intel drivers need this and they can easily check for
working agp support in their driver ->load callbacks.

This is the only reason why agp initialization could fail, so allows
us to rip out a bit of error handling code in the next patch.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>