drm/amdgpu: move amdgpu_num_kcq handling to a helper

Add a helper so we can set per asic default values. Also,
the module parameter is currently clamped to 8, but clamp it
per asic just in case some asics have different limits in the
future. Enable the option on gfx6,7 as well for consistency.

Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>