drm/i915: get rid of primary_enabled and use atomic state

This was an optimization from way back before we had primary plane
support to be able to disable the primary plane. But with primary
plane support userspace can tell the kernel this directly, so there's
no big need for this any more. And it's getting in the way of the
atomic conversion.

If need be we can resurrect this later on properly again.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
[danvet: Explain why removing this is ok.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>