egl: Restrict multiplication in calloc arguments to use compile-time constants

egl: Restrict multiplication in calloc arguments to use compile-time constants

As explained in the previous commit, we want to avoid the possibility of
integer-multiplication overflow while allocating buffers.

In these two cases, the final allocation size is the product of three values:
one variable and two that are fixed constants at compile time.

In this commit, we move the explicit multiplication to involve only the
compile-time constants, preventing any overflow from that multiplication, (and
allowing calloc to catch any potential overflow from the remainining implicit
multiplication).

Reviewed-by: Matt Turner <mattst88@gmail.com>