drm/i915: fix get_core_clock_speed for G33 class desktop chips

Somehow the case for G33 got dropped while porting from ums code.
This made a 400MHz chip into a 133MHz one which resulted in the
unnecessary enabling of double wide pipe mode which in turn
screwed up the overlay code.

Nothing else (than the overlay code) seems to be affected.

This fixes fdo.org bug #24835

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>