drm/amd/display: Remove unused temp variable

Remove unused temp variable from the dmub_rb_flush_pending function by
using arithmetic to remove the loop.

The -Wunused-but-set-variable warning was pointed out by Clang with the
following warning:

drivers/gpu/drm/amd/amdgpu/../display/dmub/inc/dmub_cmd.h:2921:12: warning:
variable 'temp' set but not used [-Wunused-but-set-variable]
    uint64_t temp;
             ^

v2: squash in revert and comment update (Alex)

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>