CHROMIUM: drm/exynos: disallow vblank_disable (for now)

exynos drm does not update crtc vblank counters when crtc vblank irqs are
disabled.

EGL does not (yet) provide a "GetMscRate", so EGL applications
compute MSC rate using MSC and UCT (ie, the vblank_count and its
corresponding timestamp) values returned from by via:
  DRI2GetMSC ->
  drmWaitVBlank({.seq=0, .type=_DRM_VBLANK_RELATIVE}) ->
  ioctl(DRM_IOCTL_WAIT_VBLANK)

Since vblanks aren't counted when the crtc vblank irq is disabled, this
computed MSC rate is miscalculated.

Disallow vblank_disable until this is fixed.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>

BUG=chromium:328953
TEST=cd /sys/kernel/debug/tracing
  # ensure no screen updates are occuring
  # for example, at blank page, click the window to disable cursor blink
  F=$(grep vblank available_filter_functions); echo ${F} > set_ftrace_filter ; echo ${F} > set_graph_function
  echo function_graph > current_tracer
  echo funcgraph-abstime > trace_options
  cat trace_pipe
  => Should still see messages like this, every 16.7 ms:
225918.023949 |   0)               |  drm_handle_vblank() {
225918.023961 |   0)   1.375 us    |    drm_vblank_count_and_time();
225918.023974 |   0) + 19.084 us   |  }
225918.040647 |   0)               |  drm_handle_vblank() {
225918.040660 |   0)   1.333 us    |    drm_vblank_count_and_time();
225918.040673 |   0) + 19.500 us   |  }
TEST=tail -F /var/log/chrome/chrome
  at blank webpage click to disable blinking cursor
  wait > 300 seconds
  click in the search box to start cursor blinking...
  => Should be no FATAL sync_control_vsync_provider message like this:
 [12556:12556:0113/061218:FATAL:sync_control_vsync_provider.cc(126)] Calculated bogus refresh interval of 250562 us. Last time base of 822186893 us. Current time base of 839225114 us. Last media stream count of 30565. Current media stream count of 30633.

Change-Id: I1290bf83d7cc4e81fc6c3710413e4c602d0af512
Reviewed-on: https://chromium-review.googlesource.com/182263
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
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