drm2: Basic use of mallocarray(9).

These functions deal the same type of overflows we do with mallocarray(9).
Using our mallocarray will panic, which different from the previous
behavior (returning NULL), but neither behavior is more correct.

As a sidenote, drm_calloc_large() is not currently used at all.

Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D13835