drm/i915/gvt: adjust mem size for low resolution type

drm/i915/gvt: adjust mem size for low resolution type

From commit 723fc57adf69 ("drm/i915/gvt: add resolution definition for vGPU
type"), small type has been restricted to small resolution, so not
require larger high GM size any more. Change to smaller 384M for more
VM creation with vGPU enabled which still perform reasonable workload.

Fixes: 723fc57adf69 ("drm/i915/gvt: add resolution definition for vGPU type")
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
(cherry picked from commit bf39ec335eb8cc51b4e1c9303ef92b380d204bb1)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>