This function is supposed to be used at mode set time, so prevent
against future mistakes by adding a WARN().

Based on a patch by Paulo Zanoni, with the check extracted into a
little assert_hdmi_port_disabled helper added to make things self
documenting and move the assert stuff out of line.

[fixed up spelling goof-up while applying.]
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>