drm/i915: dirty fb operation flushsing frontbuffer

Let's do a frontbuffer flush on dirty fb.
To be used for DIRTYFB drm ioctl.

This patch solves the biggest PSR known issue, that is
missed screen updates during boot, mainly when there is a splash
screen involved like Plymouth.

Previously PSR was being invalidated by fbdev and Plymounth
was taking control with PSR yet invalidated and could get screen
updates normally. However with some atomic modeset changes
Pymouth modeset over ioctl was now causing frontbuffer flushes
making PSR gets back to work while it cannot track the
screen updates and exit properly.

By adding this flush on dirtyfb we properly track frontbuffer
writes and properly exit PSR.

Actually all mmap_wc users should call this dirty callback
in order to have a proper frontbuffer tracking.

In the future it can be extended to return 0 if the whole
screen has being flushed or the number of rects flushed
as Chris suggested.

v2: Remove ORIGIN_FB_DIRTY and use ORIGIN_GTT instead since dirty
    callback is just called after few screen updates and not on
    everyone as pointed by Daniel.

v3: Use flush instead of invalidate since flush means
    invalidate + flush and dirty means drawn had finished and
    it can be flushed.

v4: Remove PSR from subject since it is purely frontbuffer tracking
    change and that can be useful for FBC as well.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix alignment as spotted by Paulo.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>