ARM: davinci: dm644x: fix out range signal for ED

Fix the video clock setting when custom timings are used with
pclock <= 27MHz. Existing video clock selection uses PLL2 mode
which results in a 54MHz clock whereas using the MXI mode results
in a 27MHz clock (which is the one actually desired).

This bug affects the Enhanced Definition (ED) support on DM644x.
Without this patch, out-range signals errors are were observed on
the TV when viewing ED. An out-of-range signal is often caused when
the field rate is above the rate that the television will handle.

Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Cc: Sekhar Nori <nsekhar@ti.com>
[nsekhar@ti.com: reword commit message based on on-list discussion]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>