drm/i915: s/int i/enum hpd_pin pin/

Use the enum hpd_pin type when talking about HPD pins, and rename the
variable from a very nondescript 'i' to 'pin', a name we already
use in other parts of the code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705164357.28512-6-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>