libdrm isn't build with radeon support if GRAPHIC_DRIVERS is radeonsi

The package config for libdrm looks for "radeon" in the GRAPHIC_DRIVERS var which is wrong since it should be radeonsi. Otherwise Mesa will not compile, since it depends on libdrm radeon support. Since the GRAPHIC_DRIVERS var isn't used somewher, where the "radeon" could make any sense, i've removed it.