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>

drivers/gpu/drm/amd/display/dmub/inc/dmub_cmd.h

@@ -2921,6 +2921,10 @@ static inline void dmub_rb_flush_pending(const struct dmub_rb *rb)
 		uint64_t temp;
 		uint8_t i;
 
+		/* Don't remove this.
+		 * The contents need to actually be read from the ring buffer
+		 * for this function to be effective.
+		 */
 		for (i = 0; i < DMUB_RB_CMD_SIZE / sizeof(uint64_t); i++)
 			temp = *data++;