drm/amdgpu: nuke dynamic gfx scratch reg allocation

It's over a decade ago that this was actually used for more than ring and
IB tests. Just use the static register directly where needed and nuke the
now useless infrastructure.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Lang Yu <Lang.Yu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>