drm/amd/display: Avoid overflow from uint32_t to uint8_t

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Avoid overflow from uint32_t to uint8_t [WHAT & HOW] dmub_rb_cmd's ramping_boundary has size of uint8_t and it is assigned 0xFFFF. Fix it by changing it to uint8_t with value of 0xFF. This fixes 2 INTEGER_OVERFLOW issues reported by Coverity.