drm/vesadrm: Rename vesadrm_set_gamma_lut() to vesadrm_set_color_lut()

Rename vesadrm's gamma helpers in preparation of the upcoming support
for color palettes. Gamma correction and color palettes share the same
hardware features, but the driver's old naming only indicated support
for gamma LUTs.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250714151513.309475-8-tzimmermann@suse.de