drm/i915: Add support for steered register writes

Upcoming patches will need to steer writes to multicast registers as
well as reading them.

Although the setting of the 'multicast' bit should only really matter
for write operations (reads always operate in a unicast manner and give
us the result from one specific instance), Wa_22013088509 suggests that
we leave the multicast bit enabled when performing read operations, so
we follow suit here.

Cc: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220314234203.799268-4-matthew.d.roper@intel.com