Make use of mallocarray(9) in drm drivers.

[pfg]

Summary

These seem like easy cases where we can use mallocarray(9), however I am
unsure if doing such changes will increase the difficulty for future
merges.

[dumbbell]

Hi!

What is the benefit of using mallocarray() here?

[pfg]

In D13835#290226, @dumbbell wrote:

Hi!

What is the benefit of using mallocarray() here?

TBH, I don't think there is any chance of an overflow with real hardware. Perhaps in a virtualized world there may be a chance to cause an overflow on anything or with specialized hardware (like the PS4 hackers did) but that's pretty extreme.
I am trying to keep the kernel consistent using mallocarray(9) where it can be used, but I am OK if you don't want this.

The code should be eaiser to read, but it will also make merging back the code to stable a nightmare.

[pfg]

This should be a minimal and more straightforward replacement.

Note that we now panic if there is an overflow.

[pfg]

This is small enough that shouldn't affect upstream merging (also the code is very FreeBSD-specific).
It also respects the general objective of the code.

[dumbbell]

Yes, that's fine, the patch is small enough. Thank you!