drm/amdgpu: Replace kzalloc with kcalloc

Replace kzalloc() function with its 2-factor argument form, kcalloc().

This patch replaces cases of:

	kzalloc(a * b, gfp)

with:
	kcalloc(a, b, gfp)

Also, improve the coding style and the use of sizeof during
allocation by changing sizeof(struct dc_surface_update) and
sizeof(struct dc_plane_state) to sizeof(*updates) and
sizeof(*surfaces), correspondingly.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>