Framebuffer: Support variable number of framebuffers in the UI

This change defines a macro NUM_FRAME_BUFFERS to set
the desired number of framebuffers to be used by the UI,
instead of hard-coding 2 framebuffers.

Aditional logic has been  added to handle the initialization
and destruction of NUM_FRAME_BUFFERS buffers.

Change-Id: I3a4bfec3e0f453432f2ffebf084c00f574d3be46
Signed-off-by: Rodrigo Obregon <robregon@ti.com>