firmware/sysfb: Fix VESA format selection

Some legacy BIOSes report no reserved bits in their 32-bit rgb mode,
breaking the calculation of bits_per_pixel in commit f35cd3fa7729
("firmware/sysfb: Fix EFI/VESA format selection"). However they report
lfb_depth correctly for those modes. Keep the computation but
set bits_per_pixel to lfb_depth if the latter is larger.

v2 fixes the warnings from a max3() macro with arguments of different
types; split the bits_per_pixel assignment to avoid uglyfing the code
with too many typecasts.

v3 fixes space and formatting blips pointed out by Javier, and change
the bit_per_pixel assignment back to a single statement using two casts.

v4 go back to v2 and use max_t()

Signed-off-by: Pierre Asselin <[e-mail]>
Fixes: f35cd3fa7729 ("firmware/sysfb: Fix EFI/VESA format selection")
Link: https://lore.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Tested-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/firmware/sysfb_simplefb.c

···
 	 *
 	 * It's not easily possible to fix this in struct screen_info,
 	 * as this could break UAPI. The best solution is to compute
-	 * bits_per_pixel here and ignore lfb_depth. In the loop below,
+	 * bits_per_pixel from the color bits, reserved bits and
+	 * reported lfb_depth, whichever is highest.  In the loop below,
 	 * ignore simplefb formats with alpha bits, as EFI and VESA
 	 * don't specify alpha channels.
 	 */
···
 					  si->green_size + si->green_pos,
 					  si->blue_size + si->blue_pos),
 				     si->rsvd_size + si->rsvd_pos);
+		bits_per_pixel = max_t(u32, bits_per_pixel, si->lfb_depth);
 	} else {
 		bits_per_pixel = si->lfb_depth;
 	}