drm/msm/dpu: maintain hw_mdp in kms

hw_mdp block is common for displays. No need
to reserve per display.

changes in v2:
	- use IS_ERR for error checking (Jordan Crouse)

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>