drm/i915: Remove redundant device id from IS_IRONLAKE_M macro

drm/i915: Remove redundant device id from IS_IRONLAKE_M macro

IS_IRONLAKE_M can use the already defined intel_device_info.is_mobile for
this platform, so remove the instance of Ironlake's mobile device id from
the header file and replace it with an IS_MOBILE check.

v2:
 * Improved commit text. (Chris)

v3:
 * Rebased for EHL.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326074057.27833-3-tvrtko.ursulin@linux.intel.com