drm/i915/xelpd: Increase maximum watermark lines to 255

XE_LPD continues to use the same "skylake-style" watermark
programming as other recent platforms.  The only change to the watermark
calculations compared to Display12 is that XE_LPD now allows a
maximum of 255 lines vs the old limit of 31.

Due to the larger possible lines value, the corresponding bits
representing the value in PLANE_WM are also extended, so make sure we
read/write enough bits.  Let's also take this opportunity to switch over
to the REG_FIELD notation.

Bspec: 49325
Bspec: 50419
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210514153711.2359617-3-matthew.d.roper@intel.com