drm/msm/mdp5: Fix global state lock backoff

In the Linux kernel, the following vulnerability has been resolved:

drm/msm/mdp5: Fix global state lock backoff

We need to grab the lock after the early return for !hwpipe case.
Otherwise, we could have hit contention yet still returned 0.

Fixes an issue that the new CONFIG_DRM_DEBUG_MODESET_LOCK stuff flagged
in CI:

WARNING: CPU: 0 PID: 282 at drivers/gpu/drm/drm_modeset_lock.c:296 drm_modeset_lock+0xf8/0x154
Modules linked in:
CPU: 0 PID: 282 Comm: kms_cursor_lega Tainted: G W 5.19.0-rc2-15930-g875cc8bc536a #1
Hardware name: Qualcomm Technologies, Inc. DB820c (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : drm_modeset_lock+0xf8/0x154
lr : drm_atomic_get_private_obj_state+0x84/0x170
sp : ffff80000cfab6a0
x29: ffff80000cfab6a0 x28: 0000000000000000 x27: ffff000083bc4d00
x26: 0000000000000038 x25: 0000000000000000 x24: ffff80000957ca58
x23: 0000000000000000 x22: ffff000081ace080 x21: 0000000000000001
x20: ffff000081acec18 x19: ffff80000cfabb80 x18: 0000000000000038
x17: 0000000000000000 x16: 0000000000000000 x15: fffffffffffea0d0
x14: 0000000000000000 x13: 284e4f5f4e524157 x12: 5f534b434f4c5f47
x11: ffff80000a386aa8 x10: 0000000000000029 x9 : ffff80000cfab610
x8 : 0000000000000029 x7 : 0000000000000014 x6 : 0000000000000000
x5 : 0000000000000001 x4 : ffff8000081ad904 x3 : 0000000000000029
x2 : ffff0000801db4c0 x1 : ffff80000cfabb80 x0 : ffff000081aceb58
Call trace:
drm_modeset_lock+0xf8/0x154
drm_atomic_get_private_obj_state+0x84/0x170
mdp5_get_global_state+0x54/0x6c
mdp5_pipe_release+0x2c/0xd4
mdp5_plane_atomic_check+0x2ec/0x414
drm_atomic_helper_check_planes+0xd8/0x210
drm_atomic_helper_check+0x54/0xb0
...
---[ end trace 0000000000000000 ]---
drm_modeset_lock attempting to lock a contended lock without backoff:
drm_modeset_lock+0x148/0x154
mdp5_get_global_state+0x30/0x6c
mdp5_pipe_release+0x2c/0xd4
mdp5_plane_atomic_check+0x290/0x414
drm_atomic_helper_check_planes+0xd8/0x210
drm_atomic_helper_check+0x54/0xb0
drm_atomic_check_only+0x4b0/0x8f4
drm_atomic_commit+0x68/0xe0

Patchwork: https://patchwork.freedesktop.org/patch/492701/