disp: remove kernel mapping of writeback framebuffer

This change removes the fb_kmap debugfs node along with kernel
mapping of writeback framebuffer. It also replaces dma_buf_kmap
calls with dma_buf_vmap.

Change-Id: I2fbee7fa5922071f6805d8e6df540ce87056bc0b
Signed-off-by: Narendra Muppalla <NarendraM@codeaurora.org>