drm/msm/a6xx: move preempt_prepare_postamble after error check

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

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.

Patchwork: https://patchwork.freedesktop.org/patch/687659/