gpu.c: extract_grid_size: extract out extract_size
gpu.c: extract_grid_size: extract out extract_size

This should improve readability.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg,nicolasvasilache/ppcg