drm/i915/skl: Implement queue_flip

A few bits have changed in MI_DISPLAY_FLIP to accomodate the new planes.
DE_RRMR seems to have kept its plane flip bits backward compatible.

v2: Rebase on top of nightly
v3: Rebase on top of nightly (minor conflict in i915_reg.h)
v4: Remove code that is now part of intel_crtc_page_flip()
    Don't use BUG() in default:
    Use intel_crtc->unpin_work->gtt_offset
    (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>