i915/vlv: untangle integrated clock source handling v4

The global integrated clock source bit resides in DPLL B on VLV, but we
were treating it as a per-pipe resource.  It needs to be set whenever
any PLL is active, so pull setting the bit out of vlv_update_pll and
into vlv_enable_pll.  Also add a vlv_disable_pll to prevent disabling it
when pipe B shuts down.

I'm guessing on the references here, I expect this to bite any config
where multiple displays are active or displays are moved from pipe to
pipe.

v2: re-add bits in vlv_update_pll to keep from confusing the state checker
v3: use enum pipe checks (Daniel)
    set CRI clock source early (Ville)
    consistently set CRI clock source everywhere (Ville)
v4: drop unnecessary setting of bit in vlv enable pll (Ville)

References: https://bugs.freedesktop.org/show_bug.cgi?id=67245
References: https://bugs.freedesktop.org/show_bug.cgi?id=69693
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: s/1/PIPE_B/]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>