drm/i915: Use REG_BIT() & co. for AUX CH registers

Modernize the DP AUX CH register definitions with REG_BIT() & co.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230509171411.7184-1-ville.syrjala@linux.intel.com
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>