drm/i915: Define bitmasks for ilk pfit window pos/size

Define and use the bitmasks for the x/y components
of the ilk+ panel filter window pos/size registers.

Note that we stick to the full 16 bit mask even though
the actual hardware limits are lower (and somewhat
platform dependent). BDW is actually limited to
13 bits horizontal and 12 bits vertical, with the high
bits being hardwired to zero. HSW should have the same
limits as BDW. And pre-HSW should be limited to 12bits
in both directions as that's already the limit of the
transcoder timing registers. Curiously on HSW and earlier
platforms all 16 bits can actually be set, but presumably
the hardware ignores the high bits.

v2: Switch back to full 16bit masks since that's what
    we use transcoder timign regs and PIPESRC as well

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230426135019.7603-2-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>