staging: fbtft: fix potential memory leak in fbtft_framebuffer_alloc()

In the Linux kernel, the following vulnerability has been resolved:

staging: fbtft: fix potential memory leak in fbtft_framebuffer_alloc()

In the error paths after fb_info structure is successfully allocated,

the memory allocated in fb_deferred_io_init() for info->pagerefs is not

freed. Fix that by adding the cleanup function on the error path.