drm/i915: add comment about pch pll enabling rules

Atm we have a few funny issues where we enable/disable shared
pll clocks. To make it clear that we are not required to enable/
disable the pch plls together with the other pch resources (and
so should keep it running when it's used by another pipe in
a shared pll configuration) add a comment.

This note is lifted from "Graphics BSpec: vol4g North Display Engine
Registers [IVB], Display Mode Set Sequence", step 9.d. of the enable
sequence:

"Configure and enable PCH DPLL, wait for PCH DPLL warmup (Can be
done anytime before enabling PCH transcoder)."

Since fixing the pll sharing code to no longer disable shared plls
if they're still in use is more involved, let's just stick with the
comment for now.

v2: Make the comment in the code clearer, to address questions raised
by Paulo Zanoni in review.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>