drm/i915: Tidy the macro casting by using an inline function

Some of our macros we trying to convert from an drm_device to a
drm_i915_private and then use the pointer inline. This is not only
cumbersome but prone to error. Replacing it with a typesafe function
should help catch those errors in future.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Squash in fixup to correctly order static vs. inline
qualifiers, static comes first. Also fix up another offender.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>