drm/tests: Add test cases for drm_rect_intersect()

Insert parameterized tests for the drm_rect_intersect() to ensure
correctness and prevent future regressions.

Also, create a helper for testing if two drm_rects are equal.

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230418230146.461129-2-arthurgrillo@riseup.net