MLK-17537-4: drm/bridge: nwl-dsi: implement mode_valid()

MLK-17537-4: drm/bridge: nwl-dsi: implement mode_valid()

This patch removes the exported function nwl_dsi_get_bit_clock that was
used by nwl_dsi-imx driver in order to configure the phy driver speed
and move this configuration directly into the nwl-dsi driver.
This function is now used directly by nwl-dsi to verify which mode can or
cannot be supported by the DSI PHY.
Also, in nwl-dsi, add support for mode_valid and add each supported mode
into a list kept internally so that it can apply the needed
configuration (phyref rate, dsi lanes, bit-clock) later when the mode is
used.

Signed-off-by: Robert Chiras <robert.chiras@nxp.com>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@nxp.com>