drm/msm/a6xx: move preempt_prepare_postamble after error check

drm/msm/a6xx: move preempt_prepare_postamble after error check

Move the call to preempt_prepare_postamble() after verifying that
preempt_postamble_ptr is valid. If preempt_postamble_ptr is NULL,
dereferencing it in preempt_prepare_postamble() would lead to a crash.

This change avoids calling the preparation function when the
postamble allocation has failed, preventing potential NULL pointer
dereference and ensuring proper error handling.

Fixes: 50117cad0c50 ("drm/msm/a6xx: Use posamble to reset counters on preemption")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Patchwork: https://patchwork.freedesktop.org/patch/687659/
Message-ID: <20251113082839.3821867-1-alok.a.tiwari@oracle.com>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>