drm: omapdrm: dpi: Remove unneeded regulator check

The dpi_display_enable() function ensures that a VDDS_DSI regulator is
available if the DSI uses the VDDS_DSI supply. This is not needed, as a
failure to get the VDDS_DSI supply will result in a probe failure,
dpi_display_enable() will thus never be called in that case.

Remove the check, and replace tests for the FEAT_DPI_USES_VDDS_DSI
feature with a test for the regulator object.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>