drm: amdgpu: remove obsolete reference to config CHASH

Commit 04ed8459 ("drm/amdgpu: remove chash") removes the chash
architecture and its corresponding config CHASH.

There is still a reference to CHASH in the config DRM_AMDGPU in
./drivers/gpu/drm/Kconfig.

Remove this obsolete reference to config CHASH.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>