drm/i915: [sparse] don't use variable size arrays

Sparse doesn't like:
"error: bad constant expression"

Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
[danvet: apply s/drm_malloc_ab/kcalloc bikeshed. If it's small enough
for the stack, it's small enough for kmalloc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>