drm/i915: special-case dirtyfb for frontbuffer tracking

First, an introduction. We currently have two types of GTT mmaps: the
"normal" old mmap, and the WC mmap. For frontbuffer-related features
that have automatic hardware tracking, only the non-WC mmap writes are
detected by the hardware. Since inside the Kernel both are treated as
ORIGIN_GTT, any features ignoring ORIGIN_GTT because of the hardware
tracking are destined to fail.

One of the special rules defined for the WC mmaps is that the user
should call the dirtyfb IOCTL after he is done using the pointers, so
that results in an intel_fb_obj_flush() call. The problem is that the
dirtyfb is passing ORIGIN_GTT, so it is being ignored by FBC - even
though the hardware tracking is not detecing the WC mmap operations.
So in order to fix that without having to give up the automatic
hardware tracking for GTT mmaps we transform the flush operation from
dirtyfb into a special operation: ORIGIN_DIRTYFB.

This commit fixes all the kms_frontbuffer_tracking subtests that
contain "fbc" and "mmap-wc" in their names and are currently failing
(for a total of 16 subtests).

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>