drm/mgag200: Replace struct mga_fbdev with generic framebuffer emulation

This patch replaces mgag200's framebuffer console with DRM's generic
implememtation. All respective code is being removed from the driver.

The console is set up with a shadow buffer. The actual buffer object is
not permanently pinned in video ram, but just another buffer object that
the driver moves in and out of vram as necessary. The driver's function
mga_crtc_do_set_base() used to contain special handling for the framebuffer
console. With the new generic framebuffer, the driver does not need this
code an longer.

For consistency, this patch also changes the preferred framebuffer depth.
The original code used 24 bpp by default and 32 bpp for the framebuffer. As
24 bpp is not well supported by userspace anyway, setting 32 bpp as default
makes sense.

v2:
	* rely on fbdev helpers error messages
	* document changes to preferred depth
	* dirty function no longer required

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/315832/