drm/selftests: Fix build warning -Wframe-larger-than

drm/selftests: Fix build warning -Wframe-larger-than

It seems for some random configuration drm_device is bigger than 2048
bytes.
The fix is to make the mock objects static variables.

Bug reported by 0-DAY Kernel test infrastructure here:
https://lists.01.org/pipermail/kbuild-all/2018-November/054431.html

Fixes: 6ff3d9ffdcbb ("drm/selftests: Add tests for drm_internal_framebuffer_create")
Signed-off-by: Alexandru-Cosmin Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181102130103.7753-1-alexandru-cosmin.gheorghe@arm.com