drm/i915: Use kcalloc more

No buffer overflows here, but better safe than sorry.

v2:
- Fixup the sizeof conversion, I've missed the pointer deref (Jani).
- Drop the redundant GFP_ZERO, kcalloc alreads memsets (Jani).
- Use kmalloc_array for the execbuf fastpath to avoid the memset
  (Chris). I've opted to leave all other conversions as-is since they
  aren't in a fastpath and dealing with cleared memory instead of
  random garbage is just generally nicer.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Drop the contentious kmalloc_array hunk in execbuf.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>