libdrm: remove libdrm-drivers package

We already did this for poky's recipe, but not for meta-ti's version. The
libdrm-drivers package is of no real usefulness given that no machine appears
to cause all the driver packages to be built.

Fixes these errors with create-ade:

    ERROR:opkg-p2:Package "libdrm-intel" is missing (required by libdrm-drivers)
    ERROR:opkg-p2:Package "libdrm-exynos" is missing (required by libdrm-drivers)

JIRA: SB-1472

Signed-off-by: Christopher Larson <kergoth@gmail.com>