drm/i915/gen9: Get rid of redundant watermark values

Now that we've make skl_wm_levels make a little more sense, we can
remove all of the redundant wm information. Up until now we'd been
storing two copies of all of the skl watermarks: one being the
skl_pipe_wm structs, the other being the global wm struct in
drm_i915_private containing the raw register values. This is confusing
and problematic, since it means we're prone to accidentally letting the
two copies go out of sync. So, get rid of all of the functions
responsible for computing the register values and just use a single
helper, skl_write_wm_level(), to convert and write the new watermarks on
the fly.

Changes since v1:
- Fixup skl_write_wm_level()
- Fixup skl_wm_level_from_reg_val()
- Don't forget to copy *active to intel_crtc->wm.active.skl
Changes since v2:
- Fix usage of wrong cstate
Changes since v3 (by Paulo):
- Rebase

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com> (v2)
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Lyude <cpaul@redhat.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476814189-6062-1-git-send-email-paulo.r.zanoni@intel.com