drm/radeon: Prevent leak of scratch register on resume from suspend

Cards typically have 5-7 scratch registers; one of these is reserved for
rdev->rptr_save_reg. Unfortunately the reservation is done in function
r100_cp_init, which is called by all drivers except r600 - and this
function is also invoked on resume from suspend. After several resumes,
no scratch registers are free and graphics acceleration is disabled.

Dmesg then reports either:
   *ERROR* radeon: cp failed to get scratch reg (-22).
   *ERROR* radeon: cp isn't working(-22).
   radeon 0000:01:00.0: failed initializing CP (-22).
or:
   *ERROR* radeon: failed to get scratch reg (-22).
   *ERROR* radeon: failed testing IB on GFX ring (-22).
   *ERROR* ib ring test failed (-22).

The chain of calls on boot for all except r600 is:
radeon_init -> ... -> (rXXX_init) -> rXXX_startup -> r100_cp_init

The chain of calls on resume for all except r600 is:
rXXX_resume -> rXXX_startup -> r100_cp_init.

R600 correctly allocates rptr_save_reg in r600_init (ie once only, not
in resume). However moving the code into the init functions for all
drivers means touching 4 drivers. So instead, this patch just adds a
test in r100_cp_init to avoid reallocating on resume. As the rdev
structure is allocated via kzalloc in radeon_driver_load_kms, and zero
is not a valid registerid, zero safely implies not-yet-allocated.

This issue appears to have been introduced in c7eff978 (3.6.0-rcN)

Signed-off-by: Simon Kitching <skitching@vonos.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>