drm/i915: use for_each_pipe loop to assign crtc_mask

This cleanup patch makes changes to use for_each_pipe loop
during bit-mask assignment of allowed crtc with encoder.

changes:
 - use BIT(i) macro instead of (1 << i) (Chris)
changes from V2:
 - use int for consistency (Jani)
changes from V3:
 - instead use enum pipe (Ville)
changes from V4:
 - drop DP/HDMI changes, as already part of patch from ville

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180919083126.31805-1-mahesh1.kumar@intel.com