drm/i915: add i915_config.h and move relevant declarations there

We already have i915_config.c. Add the i915_config.h counterpart, and
declutter i915_drv.h in the process.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118131538.3558599-1-jani.nikula@intel.com