Build libdrm* modules with BOARD_VNDK_VERSION

They are marked as vendor module since they all are depending on libdrm,
which is a vendor lib.

Also, libdrm_platform is newly introduced as the platform version of
libdrm.

Bug: 37342627
Bug: 63741047
Test: BOARD_VNDK_VERSION=current m -j libdrm_amdgpu libdrm_etnaviv
libdrm_freedreno libdrm_intel libdrm_nouveau libdrm_radeon
libdrm_rockchip libdrm_tegra libkms modetest atomictest planetest

Change-Id: Ic1ff6fb616f406f1c5e005d3e6f6039758d62315