qxl: Fix uninitialised struct field head.surface_id

qxl: Fix uninitialised struct field head.surface_id

The surface_id struct field in head is not being initialized and
static analysis warns that this is being passed through to
dev->monitors_config->heads[i] on an assignment. Clear up this
warning by initializing it to zero.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: a6d3c4d79822 ("qxl: hook monitors_config updates into crtc, not encoder.")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210304094928.2280722-1-colin.king@canonical.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>