drm/xe/reg_sr: Remove register pool

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.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241209232739.147417-2-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>