Currently we program just DPSCNTR and DSPSTRIDE directly from the ring
interrupt handler, which is fine since the hardware guarantees that
those are update atomically. When we have atomic page flips we'll want
to be able to update also the offset registers, and then we need to use
the vblank evade mechanism to guarantee atomicity. Since that mechanism
introduces a wait, we need to do the actual register write from a work
when it is triggered by the ring interrupt.

v2: Explain the need for mmio_flip.work in the commit message (Paulo)
    Initialize the mmio_flip work in intel_crtc_init() (Paulo)
    Prevent new flips the previous flip work finishes (Paulo)
    Don't acquire modeset locks for mmio flip work

Note: Paulo had reservations about the work item leaking over a plane
disable. But insofar as we do lack these checks that issue is already
present with the existing code.
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>