drm/i915: Don't try to use the hardware frame counter with i965gm TV output

On i965gm the hardware frame counter does not work when
the TV encoder is active. So let's not try to consult
the hardware frame counter in that case. Instead we'll
fall back to the timestamp based guesstimation method
used on gen2.

Note that the pipe timings generated by the TV encoder
are also rather peculiar. Apparently the pipe wants to
run at a much higher speed (related to the oversample
clock somehow it seems) but during the vertical active
period the TV encoder stalls the pipe every few lines
to keep its speed in check. But once the vertical
blanking period is reached the pipe gets to run at full
speed. This means our vblank timestamp estimates are
suspect. Fixing all that would require quite a bit
more work. This simple fix at least avoids the nasty
vblank timeouts that are happening currently.

Curiously the frame counter works just fine on i945gm
and gm45. I don't really understand what kind of mishap
occurred with the hardware design on i965gm. Sadly
I wasn't able to find any chicken bits etc. that would
fix the frame counter :(

v2: Move the zero vs. non-zero hw counter value handling
    into i915_get_vblank_counter() (Daniel)
    Use the per-crtc maximum exclusively, leaving the
    per-device maximum at zero
v3: max_vblank_count not populated yet in intel_enable_pipe()
    use intel_crtc_max_vblank_count() instead

Cc: stable@vger.kernel.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Fixes: 51e31d49 ("drm/i915: Use generic vblank wait")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93782


Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190122125149.GE5527@ideak-desk.fi.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>