disp: msm: dp: address race condition in LM allocation

Addresses a race condition which could cause the RM to overcommiting LMs to
DP when multiple DP displays are used.

sde_rm_get_resource_info now uses the RM's mutex and assumes a null
encoder is not a built in display so that it can return an accurate
count of unreserved resources.

DP layer now maintains internal accounting of LMs in use to avoid
validating modes that have insufficient remaining resources.

Change-Id: I908c1597c1d651b6f9c9b74a34137f30087d8801
Signed-off-by: Alex Danila <quic_eadanila@quicinc.com>

msm/dp/dp_panel.h

@@ -137,6 +137,7 @@ struct dp_panel {
 	u32 pbn;
 
 	u32 dsc_blks_in_use;
+	u32 max_lm;
 	/* DRM connector assosiated with this panel */
 	struct drm_connector *connector;