drm/simpledrm: Acquire memory aperture for framebuffer

We register the simplekms device with the DRM platform helpers. A
native driver for the graphics hardware will kick-out the simpledrm
driver before taking over the device.

The original generic platform device from the simple-framebuffer boot
code will be unregistered. The native driver will use whatever native
hardware device it received.

v4:
	* convert to drm_aperture_acquire_from_firmware()
v3:
	* use platform_device_unregister() and handle detachment
	  like hot-unplug event (Daniel)
v2:
	* adapt to aperture changes
	* use drm_dev_unplug() and drm_dev_enter/exit()
	* don't split error string

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: nerdopolis <bluescreen_avenger@verizon.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-10-tzimmermann@suse.de