Refactor performance scaling related functions in MediaTek UFS driver.

Link: https://lore.kernel.org/r/20201116065054.7658-2-stanley.chu@mediatek.comReviewed-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>