drm/vc4: remove redundant assignment of variable size

The variable 'size' is being assigned a value that is never read,
the assignment is redundant and can be removed. Cleans up clang-scan
warning:

drivers/gpu/drm/vc4/vc4_bo.c:358:2: warning: Value stored to 'size'
is never read [deadcode.DeadStores]
        size = roundup(size, PAGE_SIZE);

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112232036.1182846-1-colin.i.king@gmail.com