drm/i915/dsi: Initiate frame request in cmd mode

In TE Gate mode or TE NO_GATE mode on every flip we need to set the
frame update request bit. After this bit is set transcoder hardware will
automatically send the frame data to the panel in case of TE NO_GATE
mode, where it sends after it receives the TE event in case of TE_GATE
mode. Once the frame data is sent to the panel, we see the frame counter
updating.

v2: Use intel_de_read/write

v3: remove the usage of private_flags

v4: Use icl_dsi in func names if non static,
    fix code formatting issues. (Jani)

v5: Send frame update request at the beginning of
    pipe_update_end, use crtc_state mode_flags (Ville)

v6: Add platform and dsi checks (Ville)

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200928110834.15077-1-vandita.kulkarni@intel.com