drm/radeon: fix PLL sharing on DCE6.1 (v2)

On DCE6.1 PPLL2 is exclusively available to UNIPHYA, so it should not
be taken into consideration when looking for an already enabled PLL
to be shared with other outputs.

This fixes the broken VGA port (TRAVIS DP->VGA bridge) on my Richland
based laptop, where the internal display is connected to UNIPHYA through
a TRAVIS DP->LVDS bridge.

Bug:
https://bugs.freedesktop.org/show_bug.cgi?id=78987



v2: agd: add check in radeon_get_shared_nondp_ppll as well, drop
    extra parameter.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org