drm: Add get_scanout_position() to struct drm_crtc_helper_funcs

drm: Add get_scanout_position() to struct drm_crtc_helper_funcs

The new callback get_scanout_position() reads the current location
of the scanout process. The operation is currently located in struct
drm_driver, but really belongs to the CRTC. Drivers will be converted
in separate patches.

To help with the conversion, the timestamp calculation has been
moved from drm_calc_vbltimestamp_from_scanoutpos() to
drm_crtc_vblank_helper_get_vblank_timestamp_internal(). The helper
function supports the new and old interface of get_scanout_position().
drm_calc_vbltimestamp_from_scanoutpos() remains as a wrapper around
the new function.

Callback functions return the scanout position from the CRTC. The
legacy version of the interface receives the device and pipe index,
the modern version receives a pointer to the CRTC. We keep the
legacy version until all drivers have been converted.

v4:
	* 80-character line fixes
v3:
	* refactor drm_calc_vbltimestamp_from_scanoutpos() to minimize
	  code duplication
	* define types for get_scanout_position() callbacks
v2:
	* fix logical op in drm_calc_vbltimestamp_from_scanoutpos()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-3-tzimmermann@suse.de