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

drivers/gpu/drm/i915/Makefile

@@ -12,7 +12,7 @@
 # Note the danger in using -Wall -Wextra is that when CI updates gcc we
 # will most likely get a sudden build breakage... Hopefully we will fix
 # new warnings before CI updates!
-subdir-ccflags-y := -Wall -Wextra
+subdir-ccflags-y := -Wall -Wextra -Wvla
 subdir-ccflags-y += $(call cc-disable-warning, unused-parameter)
 subdir-ccflags-y += $(call cc-disable-warning, type-limits)
 subdir-ccflags-y += $(call cc-disable-warning, missing-field-initializers)