drm/i915: replace platform flags with a platform enum

The platform flags in device info are (mostly) mutually
exclusive. Replace the flags with an enum. Add the platform enum also
for platforms that previously didn't have a flag, and give them codename
logging in dmesg.

Pineview remains an exception, the platform being G33 for that.

v2: Sort enum by gen and date

v3: rebase on geminilake enabling

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480596595-3278-1-git-send-email-jani.nikula@intel.com