drm/radeon/kms: add support for "Surround View"

"Surround View" is an option in the system bios that
enables the AMD IGP chip in conjunction with a
discrete AMD card.  However, since the IGP vbios is
part of the system bios it is not accessible via the
rom bar or the legacy vga location.  When "Surround View"
is enabled in the system bios, the system bios puts a
copy of the IGP vbios image at the start of vram.

This patch adds support for reading the vbios image out
of vram on IGP cards.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>