drm/vesadrm: Avoid NULL-ptr deref in vesadrm_pmi_cmap_write()

Only set PMI fields if the screen_info's Vesa PM segment has been
set. Vesa PMI is the power-management interface. It also provides
means to set the color palette. The interface is optional, so not
all VESA graphics cards support it. Print vesafb's warning [1] if
the hardware palette cannot be set at all.

If unsupported the field PrimaryPalette in struct vesadrm.pmi is
NULL, which results in a segmentation fault. Happens with qemu's
Cirrus emulation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 814d270b ("drm/sysfb: vesadrm: Add gamma correction")
Link: https://elixir.bootlin.com/linux/v6.15/source/drivers/video/fbdev/vesafb.c#L375

 # 1
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250617140944.142392-1-tzimmermann@suse.de