compat-drivers: fix processing pending-stable patches

Upstream commit 919b760cab4a22092987a5653df4b67db65757f3

After we got DRM subsystem support the patches for each
subsystem were separated. Stable patches then need to
be broken down as well. This fixes this and restores
the behaviour allowing us to make -s releases.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>