intel: Fix SIGSEGV in libdrm for heigth = 0 and width = 0

drm_intel_gem_bo_free() crashes because the list bo_gem->vma_list is not
yet initialised, but the error path tries to free it.

See also https://bugs.freedesktop.org/show_bug.cgi?id=75844



Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>