drm/i915/wm: use REG_FIELD_{PREP,GET} for PLANE_WM_BLOCKS_MASK

Use REG_FIELD_{PREP,GET} for completeness, and to avoid bitwise
operations with different sizes.

v2: Also use REG_FIELD_GET in skl_wm_level_from_reg_val() (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223103517.634229-1-jani.nikula@intel.com