drm/kms/radeon: Add support for precise vblank timestamping.

This patch adds new functions for use by the drm core:

.get_vblank_timestamp() provides a precise timestamp
for the end of the most recent (or current) vblank
interval of a given crtc, as needed for the DRI2
implementation of the OML_sync_control extension.

It is a thin wrapper around the drm function
drm_calc_vbltimestamp_from_scanoutpos() which does
almost all the work and is shared across drivers.

.get_scanout_position() provides the current horizontal
and vertical video scanout position and "in vblank"
status of a given crtc, as needed by the drm for use by
drm_calc_vbltimestamp_from_scanoutpos().

The function is also used by the dynamic gpu reclocking
code to determine when it is safe to reclock inside vblank.

For that purpose radeon_pm_in_vbl() is modified to
accomodate a small change in the function prototype of
the radeon_get_crtc_scanoutpos() which is hooked up to
.get_scanout_position().

This code has been tested on AVIVO hardware, a RV530
(ATI Mobility Radeon X1600) in a Intel Core-2 Duo MacBookPro
and some R600 variant (FireGL V7600) in a single cpu
AMD Athlon 64 PC.

Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>