drm_hwcomposer: Fix check commit script to ignore case and extra spaces

Job 93709 [1] failed with missing committer sign-off. Ayan has their
committer string set to "Ayan kumar halder <ayan.halder@arm.com>", and
the Signed-off-by line on the commit was
"Signed-off-by: Ayan Kumar Halder <ayan.halder@arm.com>". So grep did
what we asked it to do and did not find the SoB since the case was
incorrect.

This patch changes to case-insensitive search and while we're at it,
trims excess whitespace from both the commit body and the
committer/author name.

Finally, I've improved the error message so it's hopefully more clear
why things fail in the future.

[1]- https://gitlab.freedesktop.org/ayan.halder/drm-hwcomposer/-/jobs/93709

Signed-off-by: Sean Paul <seanpaul@chromium.org>