drm/amdgpu: Fix warnings in gmc_v11_0.c

Fix below checkpatch warnings:

WARNING: quoted string split across lines
WARNING: Prefer 'unsigned int' to bare use of 'unsigned'
WARNING: void function return statements are not generally useful
WARNING: braces {} are not necessary for any arm of this statement

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>