drm/i915/skl: Add module parameter to select edp vswing table

This provides an option to override the value set by VBT
for selecting edp Vswing Pre-emph setting table.

v2: Adding comment about this being a temporary workaround and
making the parameter read-only (Jani)
v3: Changing mode to 0400 instead of 0 (Jani)

https://bugs.freedesktop.org/show_bug.cgi?id=89554


Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>