drm/amdkfd: Use non-atomic bitmap functions when possible

All uses of the 'kfd->gtt_sa_bitmap' bitmap are protected with the
'kfd->gtt_sa_lock' mutex.

So:
   - prefer the non-atomic '__set_bit()' function
   - use the non-atomic 'bitmap_[set|clear]()' functions instead of
     equivalent 'for' loops. These functions can work on several bits at a
     time

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>