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/msm_drv.h

@@ -828,6 +828,7 @@ struct msm_mode_info {
 
 /**
  * struct msm_resource_caps_info - defines hw resources
+ * @num_lm_in_use       number of layer mixers allocated to a specified encoder
  * @num_lm              number of layer mixers available
  * @num_dsc             number of dsc available
  * @num_vdc             number of vdc available
@@ -836,6 +837,7 @@ struct msm_mode_info {
  * @max_mixer_width:    max width supported by layer mixer
  */
 struct msm_resource_caps_info {
+	uint32_t num_lm_in_use;
 	uint32_t num_lm;
 	uint32_t num_dsc;
 	uint32_t num_vdc;