drm/ttm: increase ttm pre-fault value to PMD size

drm/ttm: increase ttm pre-fault value to PMD size

ttm page fault handler ttm_bo_vm_fault_reserved() maps
TTM_BO_VM_NUM_PREFAULT more pages beforehand
due to the principle of locality.

However, on some platform the page faults are more costly, this
patch intends to increase the number of ttm pre-fault to relieve
the number of page faults.

When multiple levels of page table is supported, the new default
value would be the PMD size, similar to huge page.

Signed-off-by: Zhu Lingshan <lingshan.zhu@amd.com>
Reported-and-tested-by: Li Jingxiang <jingxiang.li@ecarxgroup.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240604084934.225738-1-lingshan.zhu@amd.com
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>