drm: qxl: Fix NULL pointer dereference at qxl_alloc_client_monitors_config

drm: qxl: Fix NULL pointer dereference at qxl_alloc_client_monitors_config

If qxl_alloc_client_monitors_config() fails to allocate
client_monitors_config then NULL pointer dereference occurs
in function qxl_display_copy_rom_client_monitors_config() after
qxl_alloc_client_monitors_config() call.

The patch adds return error from qxl_alloc_client_monitors_config()
and additional status for qxl_display_copy_rom_client_monitors_config
return value.

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

Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Link: http://patchwork.freedesktop.org/patch/msgid/20180727153058.23620-1-vasilyev@ispras.ru
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>