drm/amdkfd: clear F8_MODE for gfx950

Default F8_MODE should be OCP format on gfx950.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Amber Lin <Amber.Lin@amd.com>