drm/amd/display: Prevent premature pageflip when comitting in vblank. (v3)

Make sure we do not program a hw pageflip inside vblank 'n' iff the
atomic flip is comitted while inside the same vblank 'n'. We must
defer such a flip by one refresh cycle to vblank 'n+1', unless this
is a DRM_MODE_PAGE_FLIP_ASYNC async pageflip, which must always
execute as soon as possible.

Without this, pageflips programmed via X11 GLX_OML_sync_control extensions
glXSwapBuffersMscOML(..., target_msc, ...); call and/or via DRI3/Present
PresentPixmap(..., target_msc, ...); request will complete one vblank
too early whenever target_msc > current_msc + 1, ie. more than 1 vblank
in the future. In such a case, the call of the pageflip ioctl() would be
triggered by a queued drmWaitVblank() vblank event, which itself gets
dispatched inside the vblank one frame before the target_msc vblank.

Testing with this patch does no longer show any problems with
OML_sync_control swap scheduling or flip completion timestamps.
Tested on R9 380 Tonga.

v2: Add acked/r-b by Harry and Michel.
v3: Feedback from Andrey: Must not wait an extra frame for
    DRM_MODE_PAGE_FLIP_ASYNC flips.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Acked-by: Harry Wentland <harry.wentland@amd.com> (v1)
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>

Cc: Harry Wentland <Harry.Wentland@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>