drm/i915: Work around GCC anonymous union initialization bug

GCC 4.4 can't cope with anonymous union initializers which seems to be a
bug in that version (see the Reference) and is fixed since GCC version
4.6. A workaround which is also used elsewhere in the kernel for the
same purpose is to wrap the initialization in curly braces, so do the
same here.

Fixes: b5565a2e ("drm/i915/bxt, glk: Give a proper name to the power well struct phy field")
Reference: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10676


Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170814151530.24154-1-imre.deak@intel.com