drm: Nuke mode->hsync

Let's just calculate the hsync rate on demand. No point in wasting
space storing it and risking the cached value getting out of sync
with reality.

v2: Move drm_mode_hsync() next to its only users
    Drop the TODO

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-2-ville.syrjala@linux.intel.com