drm/i915: Fix up pixel_rate vs. clock confusion in wm calculations

drm/i915: Fix up pixel_rate vs. clock confusion in wm calculations

Use pixel_rate rather than crtc_clock in the watermark calculations.
These are actually identical on gmch platforms for now since
we don't adjust the pixel rate based on pfit downscaling. But
pixel_rate is the thing we are actually interested here so use
the proper name for it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211209144311.3221-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>