drm: s/drm_crtc_get_hv_timings/drm_mode_get_hv_timings/

The function operates on modes, not CRTCs. Also move it into
drm_modes.[hc]. Spotted while reviewing CRTC docs.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-15-daniel.vetter@ffwll.ch