lcd: rename bitmap_plot to better represent its functionality

The name "bitmap_plot" is misleading because it implies that this is a generic
function capable of dealing with any bitmap, but its implementation only works
with the logo data.

Rename the function to better reflect this.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Anatolij Gustschin <agust@denx.de>