drm/i915: Implement WaPixelRepeatModeFixForC0:chv

drm/i915: Implement WaPixelRepeatModeFixForC0:chv

DPLL_MD(PIPE_C) is AWOL on CHV. Instead of fixing it someone added
chicken bits to propagate the pixel multiplier from DPLL_MD(PIPE_B)
to either pipe B or C. So do that to make pixel repeat work on pipes
B and C. Pipe A is fine without any tricks.

Fortunately the pixel repeat propagation appears to be a oneshot
operation, so once the value has been written we can clear the
chicken bits. So it is still possible to drive pipe B and C with
different pixel multipliers simultaneosly.

Looks like DPLL_VGA_MODE_DIS must also be set in DPLL(PIPE_B)
for this to work. But since we keep that bit always set in all
DPLLs there's no problem.

This of course means we can't reliably read out the pixel multiplier
for pipes B and C. That would make the state checker unhappy, so I
added shadow copies of those registers in to dev_priv. The other
option would have been to skip pixel multiplier, dpll_md an dotclock
checks entirely on CHV, but that feels like a serious loss of cross
checking, so just pretending that we have working DPLL MD registers
seemed better. Obviously with the shadow copies we can't detect if
the pixel multiplier was properly configured, nor can we take over
its state from the BIOS, but hopefully people won't have displays
that would be limitd to such crappy modes.

There is one strange flicker still remaining. It's visible on
pipe C/HDMID when HDMIB is enabled while driven by pipe B.
It doesn't occur if pipe A drives HDMIB, nor is there any glitch
on pipe B/HDMIB when port C/HDMID starts up. I don't have a board
with HDMIC so not sure if it happens there too. So I'm not sure
if it's somehow tied in with this strange linkage between pipe B
and C. Sadly I was unable to find an enable sequence that would
avoid the glitch, but at least it's not fatal ie. the output
recovers afterwards.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458052809-23426-4-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>