drm/i915: Add for_each_pipe_masked()

for_each_pipe_masked() can be used to iterate over the pipes
included in the user provided pipe mask. Removes a few lines of
duplicated code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455907651-16397-2-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>