drm/amd/display: remove unnecessary cast and use kcalloc instead of kzalloc

Use kcalloc instead of kzalloc and the cast on the return from kzalloc is
unnecessary and can be removed.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>