drm/i915: Polish WM_LINETIME register stuff

Let's store the normal and IPS linetime watermarks individually,
and while at it we'll pimp the register definitions as well.

v2: Deal with gvt

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200120174728.21095-2-ville.syrjala@linux.intel.com