drm/i915: use pointer = k[cmz...]alloc(sizeof(*pointer), ...) pattern

Done while reviewing all our allocations for fubar. Also a few errant
cases of lacking () for the sizeof operator - just a bit of OCD.

I've left out all the conversions that also should use kcalloc from
this patch  (it's only 2).

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>