drm/i915: Don't use enums for hardware engine id

Generally we are using macros for any hardware identifiers as these
may change between Gens. Do the same with hardware engine ids.

v2: move hw engine defs to i915_reg.h (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301202615.118632-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>