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

This reverts commit 0ddd2ae5.

This patch causes sluggishness and stuttering in graphical
apps.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3564
Link: https://www.spinics.net/lists/dri-devel/msg457005.html


Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Zhu Lingshan <lingshan.zhu@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240820134600.1909370-1-alexander.deucher@amd.com