drm/i915/skl: Read the pipe WM HW state

This patch provides the implementation for reading the pipe wm HW
state.

v2: Incorporated Damien's review comments and also made modifications
    to incorporate the plane/cursor split.

v3: No need to ident a line that was fitting 80 chars
    Return early instead of indenting the remaining of a function
    (Damien)

v4: Rebase on top of nightly (minor conflict in intel_drv.h)
v5: Rebase on top of nightly (minor conflict in intel_drv.h)
v6: Rebase on top of nightly (minor conflict in intel_drv.h)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Pradeep Bhat <pradeep.bhat@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>