drm/radeon/kms: increase rom size for atrm method

The vbios rom is >64k on a lot of modern asics.  Increase
the fetch size for atrm to make sure we don't miss part
of a larger rom.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>