drm/amdgpu: Move vram, gtt & flash defines to amdgpu_ ttm & _psp.h

As amdgpu.h is getting decomposed, move vram and gtt extern defines into
amdgpu_ttm.h & flash extern to amdgpu_psp.h

Fixes: f9acfafc ("drm/amdgpu: Move externs to amdgpu.h file from amdgpu_drv.c")
Suggested-by: Christian König <christian.koenig@amd.com>
Cc: Mario Limonciello <mario.limonciello@amd.com>
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: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>