staging: fbtft: core: Don't set device platform_data

Pass platform_data as an argument to fbtft_framebuffer_alloc()
instead of using dev->platform_data.

This fixes an issue where the device comes from Device Tree
and fbtft_probe_common() sets dev->platform_data to allocated
memory. When the module is reloaded, dev->platform_data points
to freed memory.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>