drm/fb_helper: Use min_t() to handle size_t and unsigned long

On arm32:

    drivers/gpu/drm/drm_fb_helper.c: In function ‘fb_read_screen_base’:
    include/linux/minmax.h:18:28: warning: comparison of distinct pointer types lacks a cast
    ...
    drivers/gpu/drm/drm_fb_helper.c:2041:22: note: in expansion of macro ‘min’
     2041 |  size_t alloc_size = min(count, PAGE_SIZE);
	  |                      ^~~
    drivers/gpu/drm/drm_fb_helper.c: In function ‘fb_write_screen_base’:
    include/linux/minmax.h:18:28: warning: comparison of distinct pointer types lacks a cast
    ...
    drivers/gpu/drm/drm_fb_helper.c:2115:22: note: in expansion of macro ‘min’
     2115 |  size_t alloc_size = min(count, PAGE_SIZE);
	  |                      ^~~

Indeed, on 32-bit size_t is "unsigned int", not "unsigned long".

Fixes: 222ec45f

 ("drm/fb_helper: Support framebuffers in I/O memory")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110090119.2667326-1-geert+renesas@glider.be