drm/radeon: Don't hang in radeon_flip_work_func on disabled crtc. (v2)

drm/radeon: Don't hang in radeon_flip_work_func on disabled crtc. (v2)

This fixes a regression introduced in Linux 4.4.

Limit the amount of time radeon_flip_work_func can
delay programming a page flip, by both limiting the
maximum amount of time per wait cycle and the maximum
number of wait cycles. Continue the flip if the limit
is exceeded, even if that may result in a visual or
timing glitch.

This is to prevent a hang of page flips, as reported
in fdo bug #93746: Disconnecting a DisplayPort display
in parallel to a kms pageflip getting queued can cause
the following hang of page flips and thereby an unusable
desktop:

1. kms pageflip ioctl() queues pageflip -> queues execution
   of radeon_flip_work_func.

2. Hotunplug of display causes the driver to DPMS OFF
   the unplugged display. Display engine shuts down,
   scanout no longer moves, but stays at its resting
   position at start line of vblank.

3. radeon_flip_work_func executes while crtc is off, and
   due to the non-moving scanout position, the new flip
   delay code introduced into Linux 4.4 by
   commit 5b5561b3660d ("drm/radeon: Fixup hw vblank counter/ts..")
   enters an infinite wait loop.

4. After reconnecting the display, the pageflip continues
   to hang in 3. and the display doesn't update its view
   of the desktop.

This patch fixes the Linux 4.4 regression from fdo bug #93746

<https://bugs.freedesktop.org/show_bug.cgi?id=93746>

v2: Skip wait immediately if !radeon_crtc->enabled, as
    suggested by Michel.

Reported-by: Bernd Steinhauser <linux@bernd-steinhauser.de>
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Tested-by: Bernd Steinhauser <linux@bernd-steinhauser.de>
Cc: <stable@vger.kernel.org> # 4.4+
Cc: Michel Dänzer <michel.daenzer@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>