drm/i915: rename intel_sideband.[ch] to intel_sbi.[ch]

Now that intel_sideband.[ch] has been decluttered, it's pure lpt/wpt
iosf sideband. Let's call it intel_sbi, following the function naming.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/183423ff23b2d259e4a197e74daf6bcd750bfe14.1634207064.git.jani.nikula@intel.com