drm/amdgpu: Fix error & warnings in gmc_v8_0.c

Fix below checkpatch error & warnings:

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

WARNING: braces {} are not necessary for single statement blocks
WARNING: braces {} are not necessary for any arm of this statement
WARNING: Block comments should align the * on each line

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>