drm/amd/amdgpu: Fix potential ioremap() memory leaks in amdgpu_device_init()

This ensures that the memory mapped by ioremap for adev->rmmio, is
properly handled in amdgpu_device_init(). If the function exits early
due to an error, the memory is unmapped. If the function completes
successfully, the memory remains mapped.

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:4337 amdgpu_device_init() warn: 'adev->rmmio' from ioremap() not released on lines: 4035,4045,4051,4058,4068,4337

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: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>