drm/display: use ERR_PTR on DP tunnel manager creation fail

Instead of returning a generic NULL on error from
drm_dp_tunnel_mgr_create(), use error pointers with informative codes
to align the function with stub that is executed when
CONFIG_DRM_DISPLAY_DP_TUNNEL is unset. This will also trigger IS_ERR()
in current caller (intel_dp_tunnerl_mgr_init()) instead of bypassing it
via NULL pointer.

v2: use error codes inside drm_dp_tunnel_mgr_create() instead of handling
 on caller's side (Michal, Imre)

v3: fixup commit message and add "CC"/"Fixes" lines (Andi),
 mention aligning function code with stub

Fixes: 91888b5b ("drm/i915/dp: Add support for DP tunnel BW allocation")
Cc: Imre Deak <imre.deak@intel.com>
Cc: <stable@vger.kernel.org> # v6.9+
Signed-off-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7q4fpnmmztmchczjewgm6igy55qt6jsm7tfd4fl4ucfq6yg2oy@q4lxtsu6445c