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/platformminigbm.cpp b/platformminigbm.cpp
index 24112aa..2040768 100644
--- a/platformminigbm.cpp
+++ b/platformminigbm.cpp
@@ -85,6 +85,7 @@
   bo->hal_format = gr_handle->droid_format;
   bo->format = gr_handle->format;
   bo->usage = gr_handle->usage;
+  bo->pixel_stride = gr_handle->pixel_stride;
   bo->pitches[0] = gr_handle->strides[0];
   bo->offsets[0] = gr_handle->offsets[0];
   bo->gem_handles[0] = gem_handle;