drm/i915/uncore: add intel_uncore_regs() helper

Add a helper for accessing uncore->regs instead of doing it
directly. This will help display code reuse with the xe driver.

Cc: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230627095128.208071-1-jani.nikula@intel.com