drm/vc4: Allow DBLCLK modes even if horz timing is odd.

The 2711 pixel valve can't produce odd horizontal timings, and
checks were added to vc4_hdmi_encoder_atomic_check and
vc4_hdmi_encoder_mode_valid to filter out/block selection of
such modes.

Modes with DRM_MODE_FLAG_DBLCLK double all the horizontal timing
values before programming them into the PV. The PV values,
therefore, can not be odd, and so the modes can be supported.

Amend the filtering appropriately.

See https://github.com/raspberrypi/linux/issues/4307



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>