travis: correct libdrm required regex to also track libdrm itself

travis: correct libdrm required regex to also track libdrm itself

The current regex was tracking only the libdrm_foo packages, while with
recent changed we bumped only (and rightfully so) libdrm.

Fix the regex to track any libdrm package.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>