package/libdrm: add optional cunit dependency

package/libdrm: add optional cunit dependency

Previously the option to install tests would result in the test cases
that don't have a cunit dependency, to build and be installed.

This patch adds an optional dependency on cunit so that all test cases
can be built and installed to target.

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Thomas: keep as an optional dependency, as cunit is only useful for
additional tests specific to the amdgpu backend]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>