nvc0: add support for accelerated video decoding through the dedicated engines

Currently the use of external firmware is required, with kernel and
userspace firmware needed for all Fermi cards except nvd9. Kepler and nvd9
should only require kernel firmware.
diff --git a/configure.ac b/configure.ac
index 99a08fd..ba4c203 100644
--- a/configure.ac
+++ b/configure.ac
@@ -33,7 +33,7 @@
 LIBDRM_RADEON_REQUIRED=2.4.40
 LIBDRM_INTEL_REQUIRED=2.4.38
 LIBDRM_NVVIEUX_REQUIRED=2.4.33
-LIBDRM_NOUVEAU_REQUIRED=2.4.33
+LIBDRM_NOUVEAU_REQUIRED="2.4.33 libdrm >= 2.4.41"
 DRI2PROTO_REQUIRED=2.6
 GLPROTO_REQUIRED=1.4.14
 LIBDRM_XORG_REQUIRED=2.4.24