i915: Remove silly test

drv_priv->gmbus is an array. Comparing it with NULL is somewhat less useful
than a chocolate teapot.

Possibly we should be testing bus != NULL each iteration of the loop
instead ?

gcc could help by warning too!

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>