drm/sti/sti_hdmi: convert to using is_hdmi from display info

Prefer the parsed results for is_hdmi in display info over calling
drm_detect_hdmi_monitor(). Remove the now redundant hdmi_monitor member
from struct sti_hdmi.

Cc: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Alain Volmat <avolmat@me.com>
Link: https://patchwork.freedesktop.org/patch/msgid/74fbd7a83712009734534d92e5499d4d87f0c53b.1662036058.git.jani.nikula@intel.com