drm/radeon/kms/evergreen: set accel_enabled

This is needed to enable accel in the ddx.  However,
due to a bug in older versions of the ddx, it relies
on accel being disabled in order to load properly on
evergreen chips.  To maintain compatility, we add a new
get accel param and call that from the ddx.  The old one
always returns false for evergreen cards.

[this fixes a regression with older userspaces on newer kernels].

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