drm/radeon: Use pflip irqs for pageflip completion if possible. (v2)

Skip the "manual" pageflip completion checks via polling and
guessing in the vblank handler radeon_crtc_handle_vblank() on
asics which are known to reliably support hw pageflip completion
irqs. Those pflip irqs are a more reliable and race-free method
of handling pageflip completion detection, whereas the "classic"
polling method has some small races in combination with dpm on,
and with the reworked pageflip implementation since Linux 3.16.

On old asics without pflip irqs, the classic method is used.

On asics with known good pflip irqs, only pflip irqs are used
by default, but a new module parameter "use_pflipirqs" allows to
override this in case we encounter asics in the wild with
unreliable or faulty pflip irqs. A module parameter of 0 allows
to use the classic method only in such a case. A parameter of 1
allows to use both classic method and pflip irqs as additional
band-aid to avoid some small races which could happen with the
classic method alone. The setting 1 gives Linux 3.16 behaviour.

Hw pflip irqs are available since R600.

Tested on DCE-4, AMD Cedar - FirePro 2270.

v2:  agd5f: only enable pflip interrupts on DCE4+ as they are not
reliable on older asics.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>