vmlfb: Fix error handling in cr_pll_init()

There is an error path, where iomemory is left mapped.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Alan Hourihane <alanh@fairlite.demon.co.uk>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>