Platforms/RaspberryPi: Fix build error in DisplayDxe

Commit 0c2af049 added mDriverBinding
extern module global, but did not remove the STATIC declaration, which
caused the build to break. Fix the build error by removing STATIC for
that module global variable.

Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Andrei Warkentin <andrey.warkentin@gmail.com>