drm/i915: consistent struct device naming

We currently have a mix of struct device *device, struct device *kdev,
and struct device *dev (the latter forcing us to refer to
struct drm_device as something else than the normal dev).

To simplify things, always use kdev when referring to struct device.

v2: Replace the dev_to_drm_minor() macro with the inline function
    kdev_to_drm_minor().

Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160822103245.24069-3-david.weinehall@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>