drm/kms: move driver specific fb common code to helper functions (v2)

Initially I always meant this code to be shared, but things
ran away from me before I got to it.

This refactors the i915 and radeon kms fbdev interaction layers
out into generic helpers + driver specific pieces.

It moves all the panic/sysrq enhancements to the core file,
and stores a linked list of kernel fbs. This could possibly be
improved to only store the fb which has fbcon on it for panics
etc.

radeon retains some specific codes used for a big endian
workaround.

changes:
fix oops in v1
fix freeing path for crtc_info

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>