drm/vgem: implement virtual GEM

This patch implements the virtual GEM driver with PRIME sharing which
allows vgem to import a gem object from other drivers for the purpose
of mmap-ing them to userspace. The mmap is done using the mmap
operation exported by other drivers.

v2: remove platform_device and do not attach to dma bufs
v3: use drm helpers for get/put pages
v4: correct dumb create pitch

Reviewed-by: Rob Clark <robdclark@gmail.com> (v3)
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org> (v3)
Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Zach Reizner <zachr@google.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/Kconfig

@@ -165,6 +165,15 @@ config DRM_SAVAGE
 	  Choose this option if you have a Savage3D/4/SuperSavage/Pro/Twister
 	  chipset. If M is selected the module will be called savage.
 
+config DRM_VGEM
+	tristate "Virtual GEM provider"
+	depends on DRM
+	help
+	  Choose this option to get a virtual graphics memory manager,
+	  as used by Mesa's software renderer for enhanced performance.
+	  If M is selected the module will be called vgem.
+
+
 source "drivers/gpu/drm/exynos/Kconfig"
 
 source "drivers/gpu/drm/rockchip/Kconfig"