drm/i915: Sanitycheck MMIO access early in driver load

We occasionally see the PCI device in a non-accessible state at the
point the driver is loaded.  When this happens, all BAR accesses will
read back as 0xFFFFFFFF.  Rather than reading registers and
misinterpreting their (invalid) values, let's specifically check for
0xFFFFFFFF in a register that cannot have that value to see if the
device is accessible.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230327195547.356584-2-andi.shyti@linux.intel.com