drm/fb-helper: Add multi buffer support for cma fbdev

This patch add a config to support to create multi buffer for cma fbdev.
Such as double buffer and triple buffer.

Cma fbdev is convient to add a legency fbdev. And still many Android
devices use fbdev now and at least double buffer is needed for these
Android devices, so that a buffer flip can be operated. It will need
some time for Android device vendors to abondon legency fbdev. So multi
buffer for fbdev is needed.

Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
[s.christ@phytec.de: Picking patch from
                     https://lkml.org/lkml/2015/9/14/188]
Signed-off-by: Stefan Christ <s.christ@phytec.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/075ffb50cc16ab055b5d47b30163401bb356ab51.1487175046.git-series.maxime.ripard@free-electrons.com
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 88e01e08e..78d7fc0 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -99,6 +99,15 @@
 
 	  If in doubt, say "Y".
 
+config DRM_FBDEV_OVERALLOC
+	int "Overallocation of the fbdev buffer"
+	depends on DRM_FBDEV_EMULATION
+	default 100
+	help
+	  Defines the fbdev buffer overallocation in percent. Default
+	  is 100. Typical values for double buffering will be 200,
+	  triple buffering 300.
+
 config DRM_LOAD_EDID_FIRMWARE
 	bool "Allow to specify an EDID data set instead of probing for it"
 	depends on DRM_KMS_HELPER