[libomptarget] [amdgpu] After a kernel dispatch packet is published, its contents...

[libomptarget] [amdgpu] After a kernel dispatch packet is published, its contents must not be accessed.

Fixes: SWDEV-275232 (With contributions from Ammar Elwazir, Laurent Morichetti, and Tony Tye)

The current code is racy. After the packet is submitted, the GPU will increment the read index. If this wraps around before the memory is read from it'll refer to a signal from an unrelated packet. Change avoids reading from the packet post-submission.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D110679