drm/i915/gvt: fix high-order allocation failure on late load

If the module happens to be loaded later at runtime there is a chance
memory is already fragmented enough to fail allocation of firmware
blob storage and consequently GVT init. Since it doesn't seem to be
necessary to have the blob contiguous, use vmalloc() instead to avoid
the issue.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1579723824-25711-1-git-send-email-igor.druzhinin@citrix.com