Allows gdb to access contents of user mode mapped VRAM BOs.

v2: return error for non-VRAM pools
Signed-off-by: NFelix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: NMichel Dänzer <michel.daenzer@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>