drm/bridge: waveshare-dsi: 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 and
turn proper error code on failure by using PTR_ERR().

Fixes: dbdea37a ("drm: bridge: Add waveshare DSI2DPI unit driver")
Signed-off-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250806084121.510207-1-victor.liu@nxp.com