drm/i915: Move encoder variable to tighter scope

Let's not pollute the function scope with variables when they're
only needed inside some loops.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200115190813.17971-5-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>