sr: Fix overlay harder

 * Use feature test macros instead of QCOM_BSP/MSM_BSP macros:
   - All QCOM targets require overlay and ION.  Remove stub funcs.
   - Split display is detected via MDSS_MDP_RIGHT_MIXER.
 * Alloc ION with ION_SYSTEM_HEAP_ID, as older devices don't have
   ION_SYSTEM_CONTIG_HEAP_ID defined.
 * Put gr_draw back in .bss, it does not need to be on the heap.

Tested and working on p930, mako, e980, find7

Change-Id: I1ec1a70b5d3cb40ee29ad5ae556399bf48bd825b