radeonsi: stop using the VM_ALWAYS_VALID flag

Allocation all the bo as ALWAYS_VALID means they must all fit in memory
(vram + gtt) at each command submission.
This causes some trouble when the total allocated memory is greater than
the available memory.

Possible solutions:
- being able to tag/untag a bo as ALWAYS_VALID: would require kernel changes
- disable VM_ALWAYS_VALID when memory usage is more than a percentage of the
  available memory
- disable VM_ALWAYS_VALID entirely

v1 of this patch implemented option 2. v2 (this version) implements option 3.

Related issues:
 - drm/amd#607
 - mesa/mesa#1257

It also helps with some piglit tests (-t maxsize -t "max[_-].*size" -t maxuniformblocksize):
instead of crashing the machine, the tests fail cleanly.

(cherry-pick from ab54624d)

Tested-by: Marge Bot <mesa/mesa!3709>
Part-of: <mesa/mesa!3709>