drm/i915: Use i2c bit banging instead of GMBUS

There are several reported instances of GMBUS failing to successfully
read the EDID, so revert back to bit banging until the issue is
resolved.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30371


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>