configure.ac: update libdrm amdgpu version requirement to 2.4.81

VCN decode has a new interface, and that depends on the latest libdrm

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/configure.ac b/configure.ac
index 1da67f5..b57be07 100644
--- a/configure.ac
+++ b/configure.ac
@@ -74,7 +74,7 @@
 # in the first entry.
 LIBDRM_REQUIRED=2.4.75
 LIBDRM_RADEON_REQUIRED=2.4.71
-LIBDRM_AMDGPU_REQUIRED=2.4.79
+LIBDRM_AMDGPU_REQUIRED=2.4.81
 LIBDRM_INTEL_REQUIRED=2.4.75
 LIBDRM_NVVIEUX_REQUIRED=2.4.66
 LIBDRM_NOUVEAU_REQUIRED=2.4.66