MAINTAINERS: update drm bug reporting URL

The original bugzilla seems to be read-only now, linking to the gitlab
for new bugs.

Signed-off-by: Pavel Turinský <ledoian@kam.mff.cuni.cz>
Cc: trivial@kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210228163658.54962-1-ledoian@kam.mff.cuni.cz


Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>