drm/i915/display: Fix style and conventions for DP AUX regs

Fix some whitespace issues for register definitions and keep the defines
for DP_AUX_CH_CTL and DP_AUX_CH_DATA in the right place: together with
the bit definition.

While at it add a TODO entry that those defines shouldn't be using an
implicit dev_priv.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230919192128.2045154-10-lucas.demarchi@intel.com