drm/i915: Enable async flips in i915

Enable asynchronous flips in i915 for gen9+ platforms.

v2: -Async flip enablement should be a stand alone patch (Paulo)

v3: -Move the patch to the end of the series (Paulo)

v4: -Rebased.

v5: -Rebased.

v6: -Rebased.

v7: -Rebased.

v8: -Rebased.

v9: -Rebased.

v10: -Rebased.
Signed-off-by: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200921110210.21182-9-karthik.b.s@intel.com

drivers/gpu/drm/i915/display/intel_display.c

@@ -18010,6 +18010,9 @@ static void intel_mode_config_init(struct drm_i915_private *i915)
 
 	mode_config->funcs = &intel_mode_funcs;
 
+	if (INTEL_GEN(i915) >= 9)
+		mode_config->async_page_flip = true;
+
 	/*
 	 * Maximum framebuffer dimensions, chosen to match
 	 * the maximum render engine surface size on gen4+.