drm/i915: give G45 and GM45 their own platform enums

Distinguish them better.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/987709804bc8fe55475e7481fcee03e7b86b1ba3.1480520526.git.jani.nikula@intel.com