drm/i915/display: move register functions to display/

Now that all display-related functions are grouped in
i915_driver_register(), move them to display/ so we reduce the amount of
display calls from the rest of the driver.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210213042756.953007-3-lucas.demarchi@intel.com