fbdev: fbmem: implement error handling in fbmem_init()

fbmem_init() ignores all errors, while fbmem_exit() does not
check if deallocating resources are valid.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>