ARM: davinci: dm644x: fix inconsistent variable naming

Add dm644x_ prefix to names of v4l2 variables in dm644x.c
Add dm644xevm_ prefix to names of v4l2 variables in board-dm644x-evm.c.

This makes the code consistent with rest of these files.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>