drm/amdgpu: Style fixes to PRIME code documentation

* Use consistent capitalization in the description of function arguments
* Define and consistently use the BO acronym for buffer objects
* Some minor wording improvements

Signed-off-by: Vijetha Malkai <vijetha.malkai@amd.com>

[ Michel Dänzer: Made commit log more specific ]

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>