drm/i915/display: Created exclusive version of vga decode setup

Current vga arbiter implementation in i915 needs a re-design.
The current approach would cause real problems if anyone actually
needs to talk another GPU using legacy VGA resources.

The main issue is that X becomes a slideshow if it thinks there
are multiple GPUs that have VGA decoding enabled as it insists
on adjusting the VGA routing pretty much for every little operation
involving any of the GPUs.

The cleanup will be planned for i915. Meanwhile to focus on Xe
upstreaming and have a cleaner separation, the said functionality
is being moved to a different file exclusive for i915. Xe driver
will re-use rest of the display code from i915.

v2: Addressed Jani Nikula's review comments.

v3: Dropped a duplicate function (Jani)

v4: Updated commit message with reasoning as sugested by Ville.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230929074306.1533859-1-uma.shankar@intel.com