drm/i915: Add a function to mmap framebuffer obj

[ Upstream commit eaee1c08 ]

Implement i915_gem_fb_mmap() to enable fb_ops.fb_mmap()
callback for i915's framebuffer objects.

v2: add a comment why i915_gem_object_get() needed(Andi).
v3: mmap also ttm objects.

Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404143100.10452-3-nirmoy.das@intel.com


Stable-dep-of: 1ac5167b ("drm/i915/gem: Adjust vma offset for framebuffer mmap offset")
Signed-off-by: Sasha Levin <sashal@kernel.org>