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/platformdrmgeneric.h b/platformdrmgeneric.h
index ebe27b7..d46e8b0 100644
--- a/platformdrmgeneric.h
+++ b/platformdrmgeneric.h
@@ -35,6 +35,7 @@
   int ReleaseBuffer(hwc_drm_bo_t *bo) override;
 
   uint32_t ConvertHalFormatToDrm(uint32_t hal_format);
+  uint32_t DrmFormatToBitsPerPixel(uint32_t drm_format);
 
  private:
   DrmDevice *drm_;