drm/vblank: Allow dynamic per-crtc max_vblank_count

On i965gm we need to adjust max_vblank_count dynamically
depending on whether the TV encoder is used or not. To
that end add a per-crtc max_vblank_count that takes
precedence over its device wide counterpart. The driver
can now call drm_crtc_set_max_vblank_count() to configure
the per-crtc value before calling drm_vblank_on().

Also looks like there was some discussion about exynos needing
similar treatment.

v2: Drop the extra max_vblank_count!=0 check for the
    WARN(last!=current), will take care of it in i915 code (Daniel)
    WARN_ON(!inmodeset) (Daniel)
    WARN_ON(dev->max_vblank_count)
    Pimp up the docs (Daniel)

Cc: stable@vger.kernel.org
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181127182004.28885-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_device.h

@@ -240,6 +240,12 @@ struct drm_device {
 	 * races and imprecision over longer time periods, hence exposing a
 	 * hardware vblank counter is always recommended.
 	 *
+	 * This is the statically configured device wide maximum. The driver
+	 * can instead choose to use a runtime configurable per-crtc value
+	 * &drm_vblank_crtc.max_vblank_count, in which case @max_vblank_count
+	 * must be left at zero. See drm_crtc_set_max_vblank_count() on how
+	 * to use the per-crtc value.
+	 *
 	 * If non-zero, &drm_crtc_funcs.get_vblank_counter must be set.
 	 */
 	u32 max_vblank_count;