drm/i915: Fix DISP_POS_Y and DISP_HEIGHT defines

Commit 428cb15d ("drm/i915: Clean up pre-skl primary plane registers")
introduced DISP_POS_Y and DISP_HEIGHT defines but accidentally set these
their masks to REG_GENMASK(31, 0) instead of REG_GENMASK(31, 16).

This breaks the primary display pane on at least pineview machines, fix
the mask to fix the primary display pane only showing black.

Tested on an Acer One AO532h with an Intel N450 SoC.

Fixes: 428cb15d

 ("drm/i915: Clean up pre-skl primary plane registers")
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220418150936.5499-1-hdegoede@redhat.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>