msm: sde: specify a minimum rotator clock rate

Specify a 20MHz minimum clock rate for the sde rotator. The
rotator minimum clock must be greater than the VSYNC clock rate
of 19.2MHz. Specifying 20MHz will cause the system to select the
next available acceptable rate.

Change-Id: I79b7f01e31d787bfce9364262c1a3a73a42853fb
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>