drm/i915: Parametrize the dphy and other spec specific parameters

The values of these parameters will be different for differnet panel
based on dsi rate, lane count, etc. Remove the hardcodings and make
these as parameters whch will be initialized in panel specific
sub-encoder implementaion.

This will also form groundwork for planned generic panel sub-encoder
implemntation based on VBT design enhancments to support multiple panels

v2: Mask away the port_bits before use

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>