wsi/display: check alloc failure in wsi_display_alloc_connector()

wsi/display: check alloc failure in wsi_display_alloc_connector()

vulkancts test dEQP-VK.wsi.direct_drm.surface.create_simulate_oom is failing
because in wsi_display_alloc_connector() function memory allocation for
connector is not checked for return NULL. create_simulate_oom test simulates
out of memory, hence memory allocation fails for connector and later when
tried to dereference connector program will segfault.

This patch fixes the dEQP-VK.wsi.direct_drm.surface.create_simulate_oom test
segfault issue by checking if connector is NULL afer memory allocation.

Cc: mesa-stable
Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21701>