drm/i915: Add support for port enable/disable for dual link configuration

For Dual Link MIPI Panels, both Port A and Port C should be enabled
during the MIPI encoder enabling sequence. Similarly, during the
disabling sequence, both ports needs to be disabled.

v2: Used for_each_dsi_port macro instead of for loop

v3: Used intel_dsi->ports instead of dual_link var for dual link configuration check

v4: Masking of the required MIPI port bits before writing proper values

Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>