drm/amdgpu: Move externs to amdgpu.h file from amdgpu_drv.c

Fixes the following:

WARNING: externs should be avoided in .c files
+extern const struct attribute_group amdgpu_vram_mgr_attr_group;

WARNING: externs should be avoided in .c files
+extern const struct attribute_group amdgpu_gtt_mgr_attr_group;

WARNING: externs should be avoided in .c files
+extern const struct attribute_group amdgpu_flash_attr_group;

And other style fixes:

WARNING: Block comments should align the * on each line
WARNING: void function return statements are not generally useful
WARNING: braces {} are not necessary for single statement blocks

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