ci: only trigger gallium_core_file_list jobs from dri and glx frontend changes

ci: only trigger gallium_core_file_list jobs from dri and glx frontend changes

these are the only frontends which may be used by gallium drivers in ci,
so stop triggering all driver jobs when other frontends are changed since
those changes can never affect ci

<MrCooper> Not that simple unfortunately. E.g. the llvmpipe-piglit-cl job hits
           src/gallium/frontends/clover & possibly src/gallium/targets/opencl,
           many jobs hit src/gallium/{frontends,targets}/dri and probably
           src/gallium/targets/pipe-loader, lavapipe jobs hit src/gallium/{frontends,targets}/lavapipe.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11832>