meson: do not use source_root() when possible

source_root() function is deprecated in Meson version 0.56.0 because it returns the source root of the parent project if called from a subproject.

Aside from source_root() being deprecated, why making this change?

I want to use Mesa as a subproject in a project generated by command buffer "decompiler" for Freedreno (see !19444 (merged)).