drm/gem: add drm_gem_create_mmap_offset_size()

Variant of drm_gem_create_mmap_offset() which doesn't make the
assumption that virtual size and physical size (obj->size) are the same.
This is needed in omapdrm to deal with tiled buffers.  And lets us get
rid of a duplicated and slightly modified version of
drm_gem_create_mmap_offset() in omapdrm.

Signed-off-by: Rob Clark <robdclark@gmail.com>