vulkan/wsi/display: set pDisplay to NULL on error

vulkan/wsi/display: set pDisplay to NULL on error

the spec for vkGetDrmDisplayEXT says:

  "If there is no VkDisplayKHR corresponding to the connectorId on the
   physicalDevice, the returning display must be set to VK_NULL_HANDLE.
   The provided drmFd must correspond to the one owned by the physicalDevice.
   If not, the error code VK_ERROR_UNKNOWN must be returned. (...)
   The given connectorId must be a resource owned by the provided drmFd.
   If not, the error code VK_ERROR_UNKNOWN must be returned"

We were only setting the display pointer to VK_NULL_HANDLE if the provided
drmFd was valid, however, there are CTS tests checking that it is also set
to NULL when it is not.

Fixes the following test on all drivers exposing EXT_acquire_drm_display
(tested with Intel and V3DV):
dEQP-VK.wsi.acquire_drm_display.acquire_drm_display_invalid_fd

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22283>