drm/amdgpu: reduce the time of reading VBIOS

VRAM is usually marked write combined, so change ioremap mode from
noncache to write combine for reading vbios from VRAM.

This will reduce cost time of reading vbios from 188ms to 8ms.

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