drm/i915: Adjust length of MI_LOAD_REGISTER_REG

Default length value of MI_LOAD_REGISTER_REG is 1.
Also move it out of cmd-parser-only registers since we're going to use
it in i915.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
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/20190926133142.2838-3-chris@chris-wilson.co.uk