drm/i915/display: Implement fb_mmap callback function

drm/i915/display: Implement fb_mmap callback function

If stolen memory allocation fails for fbdev, the driver will
fallback to system memory. Calculation of smem_start is wrong
for such framebuffer objs if the platform comes with no gmadr or
no aperture. Solve this by adding fb_mmap callback which will
use GTT if aperture is available otherwise will use cpu to access
the framebuffer.

v2: Use to_intel_fbdev() function(Jani)

Cc: Matthew Auld <[email protected]>
Cc: Andi Shyti <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Imre Deak <[email protected]>
Signed-off-by: Nirmoy Das <[email protected]>
Reviewed-by: Andi Shyti <[email protected]>
Reviewed-by: Andrzej Hajda <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]