drm/i915/display: start high level display driver file

The only way to truly clean up intel_display.[ch] is to move stuff out
of them until there's absolutely nothing left.

Start moving the high level display driver entry points, i.e. functions
called from top level driver code only, to a new file, which we'll call
intel_display_driver.c. The intention is that there's no low-level
display code or details here. This is an in-between layer.

Initially, move intel_display_driver_register() and
intel_display_driver_unregister() there.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e42cc037881a4c6042948a34bd4a9698f9e8487c.1681465222.git.jani.nikula@intel.com