drm/amd/amdgpu: Fix errors & warnings in amdgpu_ttm.c

Fix below checkpatch insisted error & warnings:

ERROR: Macros with complex values should be enclosed in parentheses
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'
WARNING: braces {} are not necessary for single statement blocks
WARNING: Block comments use a trailing */ on a separate line
WARNING: Missing a blank line after declarations

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>