drm/i915: split out i915_reg_read_ioctl() to i915_ioctl.[ch]

Add new files i915_ioctl.[ch] to hold small ioctls that are out of place
everywhere else, and not big enough to warrant a file of their own. For
starters, it's just for i915_reg_read_ioctl() that's a bit high level
for a low level implementation that intel_uncore.[ch] is.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220120113346.3214745-1-jani.nikula@intel.com