drm/amdgpu: Fixes to returning VBIOS RAS EEPROM address

1) Generalize the function--if the user didn't set
   i2c_address, still return true/false to
   indicate whether VBIOS contains the RAS EEPROM
   address.  This function shouldn't evaluate
   whether the user set the i2c_address pointer or
   not.

2) Don't touch the caller's i2c_address, unless
   you have to--this function shouldn't have side
   effects.

3) Correctly set the function comment as a
   kernel-doc comment.

Cc: John Clements <john.clements@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Alex Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>