hwcontext_opencl: DRM to OpenCL mapping for ARM

Using cl_arm_import_memory.  Unfortunately, despite this not being a
standard extension, the function clImportMemoryARM() is not accessible
via clGetExtensionFunctionAddressForPlatform().  This means that it has
to be linked directly to the ARM OpenCL binary, so making a portable
binary is not possible as it is with all other mapping extensions.

configure

@@ -2121,6 +2121,7 @@ HAVE_LIST="
     makeinfo
     makeinfo_html
     opencl_d3d11
+    opencl_drm_arm
     opencl_dxva2
     opencl_vaapi_beignet
     opencl_vaapi_intel_media
@@ -6178,6 +6179,11 @@ if enabled_all opencl d3d11va ; then
         enable opencl_d3d11
 fi
 
+if enabled_all opencl libdrm ; then
+    check_func_headers "CL/cl_ext.h" clImportMemoryARM &&
+        enable opencl_drm_arm
+fi
+
 enabled vdpau &&
     check_cpp_condition vdpau/vdpau.h "defined VDP_DECODER_PROFILE_MPEG4_PART2_ASP" ||
     disable vdpau