drm/i915/psr: Bring back HSW/BDW PSR AUX CH registers/setup

drm/i915/psr: Bring back HSW/BDW PSR AUX CH registers/setup

Reintroduce the special PSR AUX CH setup for hsw/bdw. Not all
of it was even removed (BDW AUX data registers were left behind).
Update the code to use REG_BIT() & co. while at it.

v2: Define the SRD_AUX_CTL bits in terms of DP_AUX_CTL bits (Jouni)
    Add a comment explaining the hand rolled DPCD write (Jouni)

Cc: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230609141404.12729-6-ville.syrjala@linux.intel.com
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>