drm/amdgpu: detach ring priority from gfx priority

Currently AMDGPU_RING_PRIO_MAX is redefinition of a
max gfx hwip priority, this won't work well when we will
have a hwip with different set of priorities than gfx.
Also, HW ring priorities are different from ring priorities.

Create a global enum for ring priority levels which each
HWIP can use to define its own priority levels.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>