drm/xe/reg_sr: Remove register pool

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

drm/xe/reg_sr: Remove register pool

That pool implementation doesn't really work: if the krealloc happens to

move the memory and return another address, the entries in the xarray

become invalid, leading to use-after-free later:

BUG: KASAN: slab-use-after-free in xe_reg_sr_apply_mmio+0x570/0x760 [xe]

Read of size 4 at addr ffff8881244b2590 by task modprobe/2753

Allocated by task 2753:

kasan_save_stack+0x39/0x70

kasan_save_track+0x14/0x40

kasan_save_alloc_info+0x37/0x60

__kasan_kmalloc+0xc3/0xd0

__kmalloc_node_track_caller_noprof+0x200/0x6d0

krealloc_noprof+0x229/0x380

Simplify the code to fix the bug. A better pooling strategy may be added

back later if needed.

(cherry picked from commit e5283bd4dfecbd3335f43b62a68e24dae23f59e4)