Cannot build drm_hwc due to Android.bp / Android.mk mix between drm_hwc and libdrm

The latest drm_hwc now uses Android.bp files. However it has a dependency on libdrm which is still using Android.mk (as of the latest tag libdrm-2.4.100, and indeed git master too).

This causes the build to fail with:

FAILED: out/soong/build.ninja 
out/soong/.bootstrap/bin/soong_build  -t -b out/soong -d out/soong/build.ninja.d -o out/soong/build.ninja Android.bp
error: external/drm_hwcomposer/Android.bp:102:1: "hwcomposer.drm_minigbm" depends on undefined module "libdrm"
error: external/drm_hwcomposer/Android.bp:67:1: "drm_hwcomposer" depends on undefined module "libdrm"
error: external/drm_hwcomposer/Android.bp:94:1: "hwcomposer.drm" depends on undefined module "libdrm"

From what I have read it is not possible for a Android.bp based module to depend on a Android.mk base module (but the reverse is allowed).

How to fix this?

Converting libdrm to Android.bp would be one way but that wants to build on Android > 4.4 and Android 5 doesn't support Android.bp