drm/amd/display: Use kcalloc() instead of kzalloc()

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

Link: https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
Link: https://github.com/KSPP/linux/issues/162

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Lenko Donchev <lenko.donchev@gmail.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>