drm/amd/display: Use 2-factor allocator calls

As already done treewide, switch from open-coded multiplication to
2-factor allocation helper.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>