Consider Meson buildtype plain everywhere (including default)
Context
We recently just hit a build issue with Mesa that asserts were enabled which was worked around through !745 (merged). I had a chat about usage of Meson on dri-devel IRC channel and it boiled down to the realization that Meson is actually not suggesting using debugoptimized that we use for distro usage (https://mesonbuild.com/Running-Meson.html) but instead plain (which is the "do nothing fancy" mode) and just construct your parameters yourself.
Description
If we make sure plain is always used in all modules, they will work the same with regards to our project build flags as any other project. @valentindavid said we currently have debugoptimized but wasn't clear to me why since the default is debug. Building things shouldn't require hacking with flags in elements for common cases.
Acceptance Criteria
Meson doesn't generate debug builds by default, no other changes.