drm/i915/rps: refactor display rps support

Make the gt rps code and display irq code interact via
intel_display_rps.[ch], instead of direct access. Add no-op static
inline stubs for xe instead of having a separate build unit doing
nothing. All of this clarifies the interfaces between i915 core and
display.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/ef2a46dc8f30b72282494f54e98cb5fed7523b58.1746536745.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>