drm/i915: Remove I915_MAX_PIPES dependency for DDB allocation

Remove dependency for I915_MAX_PIPES by replacing it with
for_each_pipe() macro.

v2: use 'enum pipe pipe' instead of 'i'

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1507630626-23806-3-git-send-email-mika.kahola@intel.com