drm/vc4: hdmi: Take into account the clock doubling flag in atomic_check

Commit 63495f6b

 ("drm/vc4: hdmi: Make sure our clock rate is within
limits") was intended to compute the pixel rate to make sure we remain
within the boundaries of what the hardware can provide.

However, unlike what mode_valid was checking for, we forgot to take
into account the clock doubling flag that can be set for modes. Let's
honor that flag if it's there.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Fixes: 63495f6b

 ("drm/vc4: hdmi: Make sure our clock rate is within limits")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>