drm/amd/display: change kzalloc to kcalloc in dcn31_validate_bandwidth()

We are trying to get rid of all multiplications from allocation
functions to prevent integer overflows. Here the multiplication is
probably safe, but using kcalloc() is more appropriate and improves
readability. This patch has no effect on runtime behavior.

Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>