drm/i915/gt: Refactor mocs loops into single control macro

We repeatedly (and more so in future) use the same looping construct
over the mocs definition table to setup the register state. Refactor the
loop construct into a reusable macro.

add/remove: 2/1 grow/shrink: 1/2 up/down: 113/-330 (-217)
Function                                     old     new   delta
intel_mocs_init_engine.cold                    -      71     +71
offset                                         -      28     +28
__func__                                   17273   17287     +14
intel_mocs_init                              143     113     -30
mocs_register.isra                            91       -     -91
intel_mocs_init_engine                       503     294    -209

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191112223600.30993-3-chris@chris-wilson.co.uk