DP panel regressions in 6.8-rc1

The internal eDP display sometimes fail to show up on the Lenovo ThinkPad X13s and sc8280xp-crd since 6.8-rc1:

[    6.006236] Console: switching to colour dummy device 80x25
[    6.007542] [drm:dpu_kms_hw_init:1048] dpu hardware revision:0x80000000
[    6.007872] [drm:drm_bridge_attach [drm]] *ERROR* failed to attach bridge /soc@0/phy@88eb000 to encoder TMDS-31: -16
[    6.007934] [drm:dp_bridge_init [msm]] *ERROR* failed to attach panel bridge: -16
[    6.007983] msm_dpu ae01000.display-controller: [drm:msm_dp_modeset_init [msm]] *ERROR* failed to create dp bridge: -16
[    6.008030] [drm:_dpu_kms_initialize_displayport:588] [dpu error]modeset_init failed for DP, rc = -16
[    6.008050] [drm:_dpu_kms_setup_displays:681] [dpu error]initialize_DP failed, rc = -16
[    6.008068] [drm:dpu_kms_hw_init:1153] [dpu error]modeset init failed: -16
[    6.008388] msm_dpu ae01000.display-controller: [drm:msm_drm_kms_init [msm]] *ERROR* kms hw init failed: -16

This can also manifest itself as a NULL-pointer dereference:

[    7.339447] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000

[    7.643705] pc : drm_bridge_attach+0x70/0x1a8 [drm]
[    7.686415] lr : drm_aux_bridge_attach+0x24/0x38 [aux_bridge]

[    7.769039] Call trace:
[    7.771564]  drm_bridge_attach+0x70/0x1a8 [drm]
[    7.776234]  drm_aux_bridge_attach+0x24/0x38 [aux_bridge]
[    7.781782]  drm_bridge_attach+0x80/0x1a8 [drm]
[    7.786454]  dp_bridge_init+0xa8/0x15c [msm]
[    7.790856]  msm_dp_modeset_init+0x28/0xc4 [msm]
[    7.795617]  _dpu_kms_drm_obj_init+0x19c/0x680 [msm]
[    7.800731]  dpu_kms_hw_init+0x348/0x4c4 [msm]
[    7.805306]  msm_drm_kms_init+0x84/0x324 [msm]
[    7.809891]  msm_drm_bind+0x1d8/0x3a8 [msm]
[    7.814196]  try_to_bring_up_aggregate_device+0x1f0/0x2f8
[    7.819747]  __component_add+0xa4/0x18c
[    7.823703]  component_add+0x14/0x20
[    7.827389]  dp_display_probe+0x47c/0x568 [msm]
[    7.832052]  platform_probe+0x68/0xd8

Users are also reporting random reboots since 6.8-rc1 and I could trigger some hard resets like that when disconnecting and reconnecting an external display, which may also be related to the DP runtime pm and panel attach rework in 6.8-rc1.