minigbm: Drop drv_bo_get_stride_in_pixels() helper

minigbm: Drop drv_bo_get_stride_in_pixels() helper

Computing "stride in pixels" assumes the pixel size divides the
stride, which isn't always the case.  This is only used for the
cros_gralloc implementation, so lets move the calculation there
instead of providing a generic helper for an unhealthy concept.

BUG=822346
TEST=test_that graphics_Gbm

Change-Id: Iab7a363c5471e4bacef7df7095ef77723adf5e93
Reviewed-on: https://chromium-review.googlesource.com/996645
Commit-Ready: Kristian H. Kristensen <hoegsberg@chromium.org>
Tested-by: Kristian H. Kristensen <hoegsberg@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>