drm/amdgpu: Fix SDMA HQD destroy error on gfx_v7

A wrong register bit was examinated for checking SDMA status so it reports
false failures. This typo only appears on gfx_v7. gfx_v8 checks the correct
bit.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>