Following to "drm/nv50/pm: s/unk05/vdec/", let's rename the PLL to PLL_VDEC

PLL names are purely indicative and are based on the most important engine
it clocks.
Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>