drm/panel: novatek-nt39016: drop use of legacy drm_bus_flags

Drop use of the legacy drm_bus_flags member and use the more descriptive
_SAMPLE_ variant.
No functional change.

Note:
DRM_BUS_FLAG_PIXDATA_SAMPLE_POSEDGE = DRM_BUS_FLAG_PIXDATA_NEGEDGE

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200630180545.1132217-5-sam@ravnborg.org