drm/ttm: add bit flag TTM_OPT_FLAG_FORCE_ALLOC

set TTM_OPT_FLAG_FORCE_ALLOC when we are servicing for page
fault routine.

for ttm_mem_global_reserve if in page fault routine, allow the gtt
pages reservation always. because page fault routing already grabbed
system memory and the allowance of this exception is harmless.
Otherwise, it will trigger OOM killer.

will be used later.

v2: set the FORCE_ALLOC always
v3: minor refine

Signed-off-by: Roger He <Hongbo.He@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>