davinci: fb: update the driver in preparation for addition of power management features

Add a helper function to enable raster.  Also add one member in the
private data structure to track the current blank status, another function
pointer which takes in the platform specific callback function to control
panel power.

These updates will help in adding suspend/resume and frame buffer blank
operation features.

Signed-off-by: Chaithrika U S <chaithrika@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>