gpu: drm: mgag200: mgag200_main:- Handle error from pci_iomap

Here, pci_iomap can fail, handle this case and return -ENOMEM.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483443027-13444-1-git-send-email-arvind.yadav.cs@gmail.com