drm/i915/psr: Nuke PSR support for VLV and CHV

PSR hardware and hence the driver code for VLV and CHV deviates a lot from
their DDI counterparts. While the feature has been disabled for a long time
now, retaining support for these platforms is a maintenance burden. There
have been multiple refactoring commits to just keep the existing code for
these platforms in line with the rest. There are known issues that need to
be fixed to enable PSR on these platforms, and there is no PSR capable
platform in CI to ensure the code does not break again if we get around to
fixing the existing issues. On account of all these reasons, let's nuke
this code for now and bring it back if a need arises in the future.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511230059.19387-1-dhinakaran.pandiyan@intel.com