drm/i915: Get rid of the "sizes are 0 based" stuff

Replace the "sizes are 0 based" stuff with just straight
up -1 where needed. Less confusing all around.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201152552.7821-4-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>