drm/i915/skl: Adjust the display engine interrupts

drm/i915/skl: Adjust the display engine interrupts

To accomodate the extra planes, the bit definitions were shuffled around
a bit.

v2: Rebase on top of the for_each_pipe() change adding dev_priv as first
    argument.
v3: Rebase after yet another change int that area (done with wiggle)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>