drm/radeon: fix resume on some rs4xx boards (v2)
drm/radeon: fix resume on some rs4xx boards (v2)

Setting MC_MISC_CNTL.GART_INDEX_REG_EN causes hangs on
some boards on resume.  The systems seem to work fine
without touching this bit so leave it as is.

v2: read-modify-write the GART_INDEX_REG_EN bit.
I suspect the problem is that we are losing the other
settings in the register.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=52952

Reported-by: Ondrej Zary <ba324ca7b1c77fc20bb970d5aff6eea9377918a5@rainbow-software.org>
Tested-by: Daniel Tobias <1d02ad41dedfa64d93a8b44199836975d651b718@gmail.com>
Signed-off-by: Alex Deucher <08dc22c6156113f2deff178e35e3ed9b24d6af9e@amd.com>
Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs