MFC r249041, r249249:

drm and i915: Left-shift iic_msg.slave at creation time

This is required because, in the radeon driver, we can't left-shift in a
central place, like it was done in the i915 driver.

Reviewed by:	kib@, kan@, avg@
Tested by:	kib@, avg@,
		Olivier Cochard-Labbé <olivier@cochard.me>