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>
drm/gem: add shmem get/put page helpers

Basically just extracting some code duplicated in gma500, omapdrm, udl,
and upcoming msm driver.

Signed-off-by: Rob Clark <robdclark@gmail.com>
drm/gma500: use gem get/put page helpers

Signed-off-by: Rob Clark <robdclark@gmail.com>
drm/udl: use gem get/put page helpers

Signed-off-by: Rob Clark <robdclark@gmail.com>
drm/omap: kill omap_gem_helpers.c

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