radeon: Firmware is required for DRM and KMS on R600 onward

radeon: Firmware is required for DRM and KMS on R600 onward

radeon requires firmware/microcode for the GPU in all chips, but for
newer chips (apparently R600 'Evergreen' onward) it also expects
firmware for the memory controller and other sub-blocks.

radeon attempts to gracefully fall back and disable some features if
the firmware is not available, but becomes unstable - the framebuffer
and/or system memory may be corrupted, or the display may stay black.

Therefore, perform a basic check for the existence of
/lib/firmware/radeon when a device is probed, and abort if it is
missing, except for the pre-R600 case.

Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name radeon-firmware-is-required-for-drm-and-kms-on-r600-onward.patch