drm/i915: add dev_to_i915 helper

This will be needed by later patches, so factor it out.

No functional change.

v2:
- s/dev_to_i915_priv/dev_to_i915/ (Jani)
- don't use the helper in i915_pm_suspend (Chris)
- simplify the helper (Chris)
v3:
- remove redundant upcasting in the helper (Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>