drm: bridge/dw_hdmi: don't support any pixel doubled modes

As mentioned in the previous commit, the dw-hdmi driver does not support
pixel doubled modes at present; it does not configure the PLL correctly
for these modes.  Therefore, filter out the double-clocked modes as we
presently are unable to support them.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>