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.
This does not seem to happen if KMS is disabled, but with both KMS
and GPU acceleration disabled radeon is not doing anything useful!

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 KMS case.

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