drm/i915/intel_i2c: allocate gmbus array as part of drm_i915_private

This memory is always allocated, and it is always a fixed size, so just
allocate it along with the rest of the driver state.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>