drm/amdgpu: Fix incorrect enum type

Instead of the 'amdgpu_ring_priority_level' type,
the 'amdgpu_gfx_pipe_priority' type was used,
which is an error when setting ring priority.
This is a minor error, but may cause problems in the future.

Instead of AMDGPU_RING_PRIO_2 = 2, we can use AMDGPU_RING_PRIO_MAX = 3,
but AMDGPU_RING_PRIO_2 = 2 is used for compatibility with
AMDGPU_GFX_PIPE_PRIO_HIGH = 2, and not change the behavior of the
code.

Signed-off-by: Grigory Vasilyev <h0tc0d3@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>