drm/i915: prefer 3-letter acronym for skylake

We are currently using a mix of platform name and acronym to name the
functions. Let's prefer the acronym as it should be clear what platform
it's about and it's shorter, so it doesn't go over 80 columns in a few
cases. This converts skylake to skl where appropriate.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224084012.24241-4-lucas.demarchi@intel.com