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 <linux@rainbow-software.org>
Tested-by: Daniel Tobias <dan.g.tob@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org