drm: Document fdinfo format specification

Proposal to standardise the fdinfo text format as optionally output by DRM
drivers.

Idea is that a simple but, well defined, spec will enable generic
userspace tools to be written while at the same time avoiding a more heavy
handed approach of adding a mid-layer to DRM.

i915 implements a subset of the spec, everything apart from the memory
stats currently, and a matching intel_gpu_top tool exists.

Open is to see if AMD can migrate to using the proposed GPU utilisation
key-value pairs, or if they are not workable to see whether to go
vendor specific, or if a standardised  alternative can be found which is
workable for both drivers.

Same for the memory utilisation key-value pairs proposal.

v2:
 * Update for removal of name and pid.

v3:
 * 'Drm-driver' tag will be obtained from struct drm_driver.name. (Daniel)

v4:
 * Added drm-engine-capacity- tag.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: David M Nieto <David.Nieto@amd.com>
Cc: Christian K...