drm/msm: basic KMS driver for snapdragon

The snapdragon chips have multiple different display controllers,
depending on which chip variant/version.  (As far as I can tell, current
devices have either MDP3 or MDP4, and upcoming devices have MDSS.)  And
then external to the display controller are HDMI, DSI, etc. blocks which
may be shared across devices which have different display controller
blocks.

To more easily add support for different display controller blocks, the
display controller specific bits are split out into a "kms" module,
which provides the kms plane/crtc/encoder objects.

The external HDMI, DSI, etc. blocks are part encoder, and part connector
currently.  But I think I will pull in the drm_bridge patches from
chromeos tree, and split them into a bridge+connector, with the
registers that need to be set in modeset handled by the bridge.  This
would remove the 'msm_connector' base class.  But some things need to be
double checked to make sure I could get the correct ON/OFF sequencing..

This patch adds support for mdp4 crtc (including hw cursor), dtv encoder
(part of MDP4 block), and hdmi.

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