drm/i915: Warn against variable length arrays

VLA are strongly discouraged in the kernel due to ambiguity they impose
on the limited stack space and security concerns over manipulating the
stack frame. Add -Wvla to our compiler flags so that CI rejects them.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313004055.25411-2-chris@chris-wilson.co.uk