drm: Verify gamma/degamma LUT size

While we want to potentially support multiple different gamma/degamma
LUT sizes we can (and should) at least check that the blob length
is a multiple of the LUT entry size.

v2: s/expected_size_mod/expected_elem_size/ (Daniel)
    Add kernel doc (Daniel)
v3: s/we/were/ typo in the docs

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180315152241.7113-1-ville.syrjala@linux.intel.com