graphics: fix desktop package recovery and Weston runtime startup

[Srikanth Muppandam (smuppand)]

Improve graphics test portability and recovery across Debian, Ubuntu, and CentOS while preserving Yocto behavior.

Changes

• Add distro-specific Weston client package mappings.
• Add CentOS mappings for:
  • weston-demo
  • seatd
  • egl-utils
  • kmscube
• Start seatd.service when required for CentOS DRM-based Weston.
• Launch Weston directly when no packaged weston.service exists.
• Dynamically select the DRM device and adopt the generated Wayland socket.
• Use existing Ubuntu repositories without configuring Debian qli sources.
• Improve Qualcomm GBM backend and graphics package detection.
• Keep missing CentOS modetest optional.
• Report a clean KMSCube skip when its RPM is unavailable.
• Update Weston and KMSCube documentation.