drm/radeon: fix up pll selection on DCE5/6

drm/radeon: fix up pll selection on DCE5/6

commit 26fe45a0a76f165425f332a5aaa298f149f9db22 upstream.

Selecting ATOM_PPLL_INVALID should be equivalent as the
DCPLL or PPLL0 are already programmed for the DISPCLK, but
the preferred method is to always specify the PLL selected.
SetPixelClock will check the parameters and skip the
programming if the PLL is already set up.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
[bwh: Backported to 3.2: drop the DCE6 case]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>