Switch all instances of malloc/free in the DRM to drm_alloc, drm_free

Switch all instances of malloc/free in the DRM to drm_alloc, drm_free
and drm_calloc.

With the recent change to check overflow in drm_calloc, this means that
all allocations that require multiplication are now checked. Also use
drm_calloc() when zeroing is needed and drop the bzero/memset
afterwards.  Finally, make drm_free() check for NULL, so we don't need
to do so every time.

ok miod@, deraadt@