negative shift exponent in cirrus_colorexpand_pattern_transp_0_24()
Description of problem:
My fuzzer detected a runtime error in cirrus_colorexpand_pattern_transp_0_24()

The stack trace is:

```
../hw/display/cirrus_vga_rop2.h:216:23: runtime error: shift exponent -2 is negative
    #0 0x5589a028c89a in cirrus_colorexpand_pattern_transp_0_24 hw/display/cirrus_vga_rop2.h:216:23
    #1 0x5589a031e239 in cirrus_bitblt_common_patterncopy hw/display/cirrus_vga.c:689:5
    #2 0x5589a032735d in cirrus_bitblt_cputovideo_next hw/display/cirrus_vga.c:820:13
    #3 0x5589a032cde9 in cirrus_linear_write hw/display/cirrus_vga.c:2365:13
    #4 0x5589a2982823 in memory_region_write_accessor system/memory.c:497:5
    #5 0x5589a2981f05 in access_with_adjusted_size system/memory.c:573:18
    #6 0x5589a297fe69 in memory_region_dispatch_write system/memory.c:1521:16
    #7 0x5589a2a2193e in flatview_write_continue_step system/physmem.c:2749:18
    #8 0x5589a2a211d4 in flatview_write_continue system/physmem.c:2779:19
    #9 0x5589a29f9cfb in flatview_write system/physmem.c:2810:12
    #10 0x5589a29f97c8 in address_space_write system/physmem.c:2930:18
...
```
Steps to reproduce:
Arguments:\
export QEMU_ARGS="-display none -machine accel=qtest, -m 512M -machine\
q35 -nodefaults -device cirrus-vga -display vnc=localhost:99 -L ../pc-bios/"\
The base addresses of memory regions:

* cirrus-io, 0x3b0
* cirrus-low-memory, 0xa0000
* cirrus-linear-io, 0xe0000000
* cirrus-bitblt-mmio, 0xe1000000
* cirrus-mmio, 0xe2000000

Reproducer:

```
writeb 0xe2000108 0x642a8d58
writeb 0xe2000117 0x335af91c
writeb 0xe2000118 0x765861ed
writeb 0xe200010d 0x7c3af934
writeb 0xe2000140 0x33f13baf
clock_step
writeb 0xe01f0e68 0x6ea3696c
writeb 0xe13bc720 0x11bb09ba
readb 0xe2000133
writeb 0xe033629b 0x80f19dd
writeb 0xe134bba7 0x1eb198f9
readb 0xe2000680
writeb 0xe2000b84 0x3f0591fc
clock_step
writeb 0xe003469e 0xdbd627e
writeb 0xe114f2bc 0x41adfe48
readb 0xe2000cde
readb 0xb269d
writeb 0xe1368066 0x3c9ab77
readb 0xe12a7fe1
writeb 0xe0191988 0x7e18b0d1
EOF
```
Additional information:
Ack: Chuhong Yuan (hslester96@gmail.com)