drm/amdgpu: Fix errors & warnings in gmc_ v6_0, v7_0.c

drm/amdgpu: Fix errors & warnings in gmc_ v6_0, v7_0.c

Fix below checkpatch errors & warnings:

ERROR: trailing statements should be on next line
+       default: BUG();
ERROR: trailing statements should be on next line

WARNING: braces {} are not necessary for single statement blocks
WARNING: braces {} are not necessary for any arm of this statement
WARNING: Block comments use * on subsequent lines
WARNING: Missing a blank line after declarations
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'

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