drm/amdgpu/gfx6: drop duplicate code

The compute functions just called the gfx functions, drop
the wrapper.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>