drm/amdgpu: set default gfx reset masks for gfx6-8
These were not set so soft recovery was inadvertantly
disabled.
Fixes: 6ac55eab4fc4 ("drm/amdgpu: move reset support type checks into the caller")
Reviewed-by: Jesse Zhang <Jesse.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>