drm: Flatten drm_mode_vrefresh()

Remove the pointless whole-function indentation. Also don't
need to worry about negative values anymore since we switched
everything to u16.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-10-ville.syrjala@linux.intel.com