video/v4l2: Rename V4L2PixFmtToNumPlanes() to GetNumPlanesOfV4L2PixFmt().

We have V4L2Device::V4L2PixFmtToNumPlanes(). It has a mapping table
of V4L2PixFmt to #planes. However, via converting V4L2PixFmt to
VideoPixelFormat, we can get result using VideoFrameLayout::NumPlanes().

To avoid duplication, we remove V4L2PixFmtToNumPlanes(). And to
facilitate V4L2 code to look up number of planes, we add
GetNumPlanesOfV4L2PixFmt() in V4L2Device.

BUG: b:132589320
TEST: Pass media_unittests

Change-Id: Iab286a06247efa91cc1e8029809145dda59d99d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1651583
Commit-Queue: Shuo-Peng Liao <deanliao@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667837}