drm/i915: Don't relay on I915_MAX_PIPES

Let's remove the dependency on I915_MAX_PIPES. Instead, get the number
of pipes from platform information.

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/1507630626-23806-2-git-send-email-mika.kahola@intel.com