Audit the mesa build configs to make sure none sets `-D platforms=drm,surfaceless` (or only does so on old builds)

Since mesa/mesa!3161 (merged) a few days ago, the -D platforms options drm & surfaceless are no longer necessary. They are still accepted but are no-ops that only generate a warning message. surfaceless is now always available, and drm is automatically enabled based on the system_has_kms_drm whitelist.

The following behaviour of the CI is needed:

• up to 20.2 (inclusive) and up to master at mesa/mesa@e00adef3, these two -D platforms values must be set when needed
• starting at that commit in master, and in 20.2+, there is no need to specify these in -D platforms
• in 20.3+, and in master starting mid/late august, if they are still passed it will be an error

mesa/mesa!5844 (merged) will be gated on this issue being closed.