drm/msm: rework binding of Imageon GPUs

Currently the msm driver creates an extra interim platform device for
Imageon GPUs. This is not ideal, as the device doesn't have
corresponding OF node. If the headless mode is used for newer GPUs, then
the msm_use_mmu() function can not detect corresponding IOMMU devices.
Also the DRM device (although it's headless) is created with modesetting
flags being set.

To solve all these issues, rework the way the Imageon devices are bound.
Remove the interim device, don't register a component and instead use a
cut-down version of the normal functions to probe or remove the driver.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/662584/


Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>