drm/i915: Move {pin, long}_mask initialization to caller from intel_get_hpd_pins()

Move the 0 initialization of pin_mask and long_mask from
intel_get_hpd_pins() into each caller. This we we can call
intel_get_hpd_pins() multiple times to accumulate more pins from several
sources.

v2: Add a comment explaining the dangers of intel_get_hpd_pins() (Paulo)

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>