drm_hwcomposer: Store pixel_stride in the hwc_drm_bo

The new GraphicBufferMapper ImportBuffer takes a pixel_stride
as an argument, so we need to caluclate that and store it in
the hwc_drm_bo at import so we can use it here.

I'd still welcome better ideas to calculate pixel_stride for
generic drm importer.

Change-Id: Iea2c483f3750dd5bed38740802b560911bc54ef2
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
v2:
* Utilized pixel stride values already in some gralloc handle
  implementations
* Fixed embarasing bit/byte math snafu (preserving bits per
  pixel since YUV conversions using bytes doesn't seem sane)
    - Extra review still would be appreciated here!
v3:
* Renamed to use Drm... instead of DRM...
diff --git a/platformhisi.cpp b/platformhisi.cpp
index 1723cb8..68bb5a7 100644
--- a/platformhisi.cpp
+++ b/platformhisi.cpp
@@ -99,7 +99,7 @@
   bo->hal_format = hnd->req_format;
   bo->format = fmt;
   bo->usage = hnd->usage;
-
+  bo->pixel_stride = hnd->stride;
   bo->pitches[0] = hnd->byte_stride;
   bo->gem_handles[0] = gem_handle;
   bo->offsets[0] = 0;