drm/amdgpu: restrict the hw sched jobs number to power of two

the module parameter sched_hw_submission is probably from user mode,
and the kernel need to check whether it is legal.

1. align hw sched jobs to power of 2 and set minimum number is 2.
2. use kernel api is_power_of_2() to simplify driver code.

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>