MAINTAINERS: repair reference in DRM DRIVER FOR SIMPLE FRAMEBUFFERS

Commit 11e8f5fd ("drm: Add simpledrm driver") adds the file
./drivers/gpu/drm/tiny/simpledrm.c, but refers to the file
./drivers/gpu/drm/tiny/simplekms.c with the new MAINTAINERS section
DRM DRIVER FOR SIMPLE FRAMEBUFFERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    drivers/gpu/drm/tiny/simplekms.c

Repair the file entry by referring to the right location.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210517101648.29906-1-lukas.bulwahn@gmail.com