qxl: fix null-pointer crash during suspend

qxl: fix null-pointer crash during suspend

"crtc->helper_private" is not initialized by the QXL driver and thus the
"crtc_funcs->disable" call would crash (resulting in suspend failure).
Fix this by converting the suspend/resume functions to use the
drm_mode_config_helper_* helpers.

Tested system sleep with QEMU 3.0 using "echo mem > /sys/power/state".
During suspend the following message is visible from QEMU:

    spice/server/display-channel.c:2425:display_channel_validate_surface: canvas address is 0x7fd05da68308 for 0 (and is NULL)
    spice/server/display-channel.c:2426:display_channel_validate_surface: failed on 0

This seems to be triggered by QXL_IO_NOTIFY_CMD after
QXL_IO_DESTROY_PRIMARY_ASYNC, but aside from the warning things still
seem to work (tested with both the GTK and -spice options).

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Link: http://patchwork.freedesktop.org/patch/msgid/20180904202747.14968-1-peter@lekensteyn.nl
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>