drm/i915/gmch: split out soc/intel_gmch

The GMCH related code is a bit too low level and out of place for the
high level i915_driver.c file. Split out to a separate file under
soc/. Rename the functions accordingly.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/980a5e08b397bc0dbccf93cd84798772233ce75c.1673958757.git.jani.nikula@intel.com