drm/vc4_validate: Remove VLA usage

In preparation to enabling -Wvla, remove VLA. In this particular
case use macro ARRAY_SIZE so the length of array _bo_ can be
computed at preprocessing time.

The use of stack Variable Length Arrays needs to be avoided, as they
can be a vector for stack exhaustion, which can be both a runtime bug
or a security flaw. Also, in general, as code evolves it is easy to
lose track of how big a VLA can get. Thus, we can end up having runtime
failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313143151.GA27486@embeddedgus

drivers/gpu/drm/vc4/vc4_validate.c

@@ -753,7 +753,7 @@ validate_gl_shader_rec(struct drm_device *dev,
 		28, /* cs */
 	};
 	uint32_t shader_reloc_count = ARRAY_SIZE(shader_reloc_offsets);
-	struct drm_gem_cma_object *bo[shader_reloc_count + 8];
+	struct drm_gem_cma_object *bo[ARRAY_SIZE(shader_reloc_offsets) + 8];
 	uint32_t nr_attributes, nr_relocs, packet_size;
 	int i;