drm/radeon/kms: make sure pci max read request size is valid on evergreen+ (v2)

If the bios or OS sets the pci max read request size to 0 or an
invalid value (6,7), it can result in a hang or slowdown.  Check
and set it to something sane if it's invalid.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=42162

v2: use pci reg defines from include/linux/pci_regs.h

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>