drm/etnaviv: Fix driver unregistering

Russell King reported:

"When removing and reloading the etnaviv module, the following splat
occurs:

sysfs: cannot create duplicate filename '/devices/platform/etnaviv'
CPU: 0 PID: 1471 Comm: modprobe Not tainted 4.17.0+ #1608
Hardware name: Marvell Dove (Cubox)
Backtrace:
[<c00157d4>] (dump_backtrace) from [<c0015b8c>] (show_stack+0x18/0x1c)
 r6:ef033e38 r5:ee07b340 r4:edb9d000 r3:00000000
[<c0015b74>] (show_stack) from [<c0620784>] (dump_stack+0x20/0x28)
[<c0620764>] (dump_stack) from [<c01bcd24>] (sysfs_warn_dup+0x5c/0x70)
[<c01bccc8>] (sysfs_warn_dup) from [<c01bce14>] (sysfs_create_dir_ns+0x90/0x98)
..."

Commit 246774d1 ("drm/etnaviv: remove the need for a gpu-subsystem
DT node") introduced DRM registration via
platform_device_register_simple(), but missed to call
platform_device_unregister() inside etnaviv_exit().

Fix the problem by calling platform_device_unregister() inside
etnaviv_exit(). While at it, also rearrange the function calls
in the exit path to make them happen in the opposite order of
registration.

Tested on a imx6-sabresd board.

Cc: <stable@vger.kernel.org>
Fixes: 246774d1 ("drm/etnaviv: remove the need for a gpu-subsystem DT node")
Reported-by: Russell King <linux@armlinux.org.uk>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>