i915/dsi: Configure TE interrupt for cmd mode

Configure TE interrupt as part of the vblank enable call flow.

v2: Hide the private flags check inside configure_te (Jani)

v3: Fix the position of masking de_port_masked for DSI_TE.

v4: Simplify the caller of configure_te (Jani)

v5: Clear IIR, remove the usage of private_flags

v6: including icl_dsi header is not needed

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/20200924124209.17916-3-vandita.kulkarni@intel.com