libva_headers misses va/va_drm.h

[rosetta-jpn]

I depends on libva_headers in Android.bp. But the include path doesn't have va/va_drm.h.
In usual build, va/va_drm.h is created by moving va/drm/va_drm.h to va/va_drm.h.
I think we need to do the same thing in libva_headers.

[rosetta-jpn]

@JeevakaPrabu could you take a look? This prevents introducing cros-libva to AOSP.

@andrescj-chromium for visibility.

[rosetta-jpn]

The issue is reproducible by m libcros_libva_bindgen with chromeos/cros-libva#17.