drm/amdgpu: Merge get_reserved_allocation to get_vbios_allocations.

Some ASICs need reserved memory for firmware or other components,
which is not allowed to be used by driver.
amdgpu_gmc_get_reserved_allocation is to handle additional areas.
To avoid any missing calling,
merged amdgpu_gmc_get_reserved_allocation to
amdgpu_gmc_get_vbios_allocations.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>