drm/bridge: analogix_dp: Fix bailout for devm_drm_bridge_alloc()

devm_drm_bridge_alloc() returns ERR_PTR on failure instead of a
NULL pointer, so use IS_ERR() to check the returned pointer.
While at it, on failure, return ERR_CAST(dp) instead of
ERR_PTR(-ENOMEM) in order not to depend on devm_drm_bridge_alloc()
error code implementation.

Fixes: 48f05c3b ("drm/bridge: analogix_dp: Use devm_drm_bridge_alloc() API")
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://lore.kernel.org/r/20250806095224.527938-1-victor.liu@nxp.com