drm/amdgpu: Fix build warnings

Fix header guard and make internal functions static. Fixes the below warnings:

drivers/gpu/drm/amd/amdgpu/../amdgpu/amdgpu_reset.h:24:9: warning: '__AMDUGPU_RESET_H__' is used as a header guard here, followed by #define of a different macro [-Wheader-guard]
drivers/gpu/drm/amd/amdgpu/aldebaran.c:110:6: warning: no previous prototype for function 'aldebaran_async_reset' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/aldebaran_ppt.c:1435:5: warning: no previous prototype for function 'aldebaran_mode2_reset' [-Wmissing-prototypes]

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>