drm/amdkfd: fix resource_size.cocci warnings

Use resource_size function on resource object
instead of explicit computation.

Clean up coccicheck warning:
./drivers/gpu/drm/amd/amdkfd/kfd_migrate.c:905:10-13: ERROR: Missing
resource_size with res

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Amos Kong <kongjianjun@gmail.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>