drm/i915: Workaround VLV/CHV DSI scanline counter hardware fail

drm/i915: Workaround VLV/CHV DSI scanline counter hardware fail

The scanline counter is bonkers on VLV/CHV DSI. The scanline counter
increment is not lined up with the start of vblank like it is on
every other platform and output type. This causes problems for
both the vblank timestamping and atomic update vblank evasion.

On my FFRD8 machine at least, the scanline counter increment
happens about 1/3 of a scanline ahead of the start of vblank (which
is where all register latching happens still). That means we can't
trust the scanline counter to tell us whether we're in vblank or not
while we're on that particular line. In order to keep vblank
timestamping in working condition when called from the vblank irq,
we'll leave scanline_offset at one, which means that the entire
line containing the start of vblank is considered to be inside
the vblank.

For the vblank evasion we'll need to consider that entire line
to be bad, since we can't tell whether the registers already
got latched or not. And we can't actually use the start of vblank
interrupt to get us past that line as the interrupt would fire
too soon, and then we'd up waiting for the next start of vblank
instead. One way around that would using the frame start
interrupt instead since that wouldn't fire until the next
scanline, but that would require some bigger changes in the
interrupt code. So for simplicity we'll just poll until we get
past the bad line.

v2: Adjust the comments a bit

Cc: stable@vger.kernel.org
Cc: Jonas Aaberg <cja@gmx.net>
Tested-by: Jonas Aaberg <cja@gmx.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99086
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161215174734.28779-1-ville.syrjala@linux.intel.com
Tested-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>