drm/i915: Have pfit calculations return an error code

drm/i915: Have pfit calculations return an error code

Change intel_{gmch,pch}_panel_fitting() to return a normal
error vs. success int. We'll need this later to validate that
the margin properties aren't misconfigured.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200422161917.17389-6-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>