drm/amdgpu: select compute ME engines dynamically

GFX ME right now is one but this could change in
future SOC's. Use no of ME for GFX as start point
for ME for compute for GFX10.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>