drm/i915/fbdev: Use lmem physical addresses for fb_mmap() on discrete

Use local memory io BAR address for fbdev's fb_mmap() operation on
discrete, fbdev uses the physical address of our framebuffer for its
fb_mmap() fn.

Signed-off-by: Mohammed Khajapasha <mohammed.khajapasha@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210427085417.120246-5-matthew.auld@intel.com