drm/i915: Split GPU commands definitions into separate header

We should not mix MMIO with MI_INSTR definitions.

v2: sanitize comment, change include order (Chris)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313124109.39216-1-michal.wajdeczko@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20180313231920.6932-1-chris@chris-wilson.co.uk