drm/amdgpu: Correctly clear GCEA error status

While clearing GCEA error status, do not clear the bits
set by RAS TA.

Signed-off-by: Mukul Joshi <mukul.joshi@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>