drm/amdkfd: simplify drain retry fault

unmap range always increase atomic svms->drain_pagefaults to simplify
both parent range and child range unmap, page fault handle ignores the
retry fault if svms->drain_pagefaults is set to speed up interrupt
handling. svm_range_drain_retry_fault restart draining if another
range unmap from cpu.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>